The suitability of each type to be a first gym
- Fire/Water/Grass - as the "basic" types, they're sort of a given, though it's funny that it took until Gen VIII to actually do it - unless you count BW's three-in-one gym. But that's a gimmick it seems unlikely we'll see again.
- Electric - mmm, Grass resists it but ultimately it's too tricky for a first gym. Its sole weakness, Ground, is rarely encountered very early on. It lends itself much more to the mid-game, which is why that's usually exactly where it is.
- Bug - well, we've actually had a first gym be Bug before. And it's a lot of fun. So this one gets a thumbs up. The BST of fully-evolved Bug Pokemon like Butterfree and Vivillon is roughly comparable to that of Onix, the OG first gym signature, so they work as early bosses.
- Flying - the same thing goes for Bug. Although fully-evolved Flying-types tend to be more powerful, hence the only example being the NFE Pidgeotto. I'm curious as to whether they'd go with Flying again - they obviously wanted to do something different in GSC but it doesn't match up to the FWG trio as elegantly as Rock does.
- Fighting/Poison/Fairy/Psychic/Dark - lumping these all together for the same reason: these are all very much midgame types. They're all defensively neutral to the main three, so are not easy to defeat. Dark does tend to come relatively early (and has generally weaker Pokemon overall) so I'm not saying it couldn't happen - I just don't think it will.
- Ghost - nope. It's immune to Normal which, as mentioned, basically everything gets early on - but has no easy early-accessible weaknesses. It has the potential to be very overpowered in the early stage of the game.
- Ground - now that works, largely because it's basically the same as Rock in most regards. I suspect that the reason Rock is picked over Ground is that Ground is slightly less difficult - your typical early-game Bugs and Normals aren't disadvantaged against it
- Dragon - would be interesting, because all the starter types are disadvantaged against it. I'd actually be very interested in seeing a Dragon gym early on if Game Freak were ever minded to do it. But there is a sense that Game Freak seems to think that Dragon ought to be a late-game type, even if - as in recent games - Dragons can be caught early on. There is no precedent for the first Gym Leader of a region using a Pokemon which the player cannot obtain until much later in the game, and even basic-stage Dragons tend to be quite powerful.
- Ice - similar reasons to Dragon; Ice is always a late-game type in every game. Again, I'd like to see it but I think that it goes against Game Freak's design conventions. The freeze status might also be considered too broken for the early-game; even though it typically has a low chance of happening it could potentially be difficult to deal with for newer/younger players.
- Steel - would actually be interesting because Fire has the edge and Grass and Water are both disadvantaged. Part of me thinks that, like Ice and Dragon, Game Freak consider Steel to be a late-game type - Steel types often have very high defences and too many resistances without stacking the deck in the early stage of the game. Skarmory for instance would have been utterly dominating if Falkner used it.
- Rock/Normal - already covered.
Very interesting thoughts. It is an interesting thought question, what principles and factors goes into choosing the first Gym Type. I personally think, under the right circumstances, any Type could be the first Gym Type as long as you include a way to defeat the Gym (be it by giving a disadvantage Starter a handy coverage move early or have a Pokemon catchable nearby that can do the job). To go over some you had questions about:
Electric & Poison: If they already had covered the basics like the Type Chart before the first Gym, an Electric-type Gym I think could be a good choice as it can teach about the Status Ailments. Of the five notable Status Ailments, I think Paralysis and Poison are the most common so wouldn't feel "unfair" to focus the first Gym upon. They could even have a few Wild Pokemon that could Paralyze and Poison to get the player used to them. Though while annoying in a Wild Battle, in an actual trainer battle with a competent Gym Leader ai could show how tides of a battle can quickly turn. Luckily, being this is in-game, you can use Paralyze Heals, Antidotes, and Berries to manage the battle. Let you catch an Electric/Poison-type or even have an early Ground-type nearby would further help manage the battle (not to mention Mud-Slap is a move a lot of Pokemon learn early and comes with a handy Accuracy decrease).
Flying: The tricky thing about Flying is that its more of a counter for later Types like Fighting & Ground. If Falkner's Gym felt not quite right for a first Gym it's cause it couldn't really bring anything to the table, at least at that point in the franchise (heck, Falkner's main strategy is Mud-Slap of all moves). However I think by now a Flying-type First Gym does have enough utilities it could counter simple early game strategies forcing the player to think more on their toes. Flying-types are normally fast but also have Tailwind to assure they'll always go first, a lot of early mons are Physical focused so Feather Dance puts a hard stop on them, Aerial Ace assures they have a STAB which always hits, Pluck can put a stop to any Berry holding strategy, and Air Cutter threatens higher chances to crit. If they especially want to be cruel Flying-type have Roost for reliable healing (as well as removing their Flying-type adding another level of complexity) and if they bring Mirror Move back I can see it being used as a surprise counter. Flying can be used to show there's way of controlling the battle other than having the stronger moves & buffing your Pokemon, and how effective it can be. An early Rock- and/or Electric-type Pokemon should put the player back in control.
Fighting & Psychic: The danger with these Types is that they hit HARD. If there's anything these Types would teach is that no one Pokemon is an army; you'll need a team and it'll likely be a battle of attrition. Obviously Fighting leans on the physical side and Psychic on the special so which one is chosen will affect the way the player has to adapt and the needed tools for them to do so. With Fighting it's more likely the Wild members of your party will be carrying the weight, especially any Flying- and Bug-types. With Psychics it's likely only your Starter would have high enough Special Defense to actually stand a chance to all your other Pokemon are serving a quick role or doing chip damage (though an early Dark-type could easily change that, and Bite as well as Bug Pokemon & Moves are normally available early on as well). Hey, maybe could also be used as a teaching tool to use those opponent debuffing moves most players tend to pass over. Maybe even give the player a Chople Berry/Payapa Berry/Focus Sash to give their Pokemon a better chance of surviving at least for two turns.
Fairy & Dark: Honestly I don't see that big of a problem with these Types. Infact Dark-types aren't known for hitting hard (on the surface), they're known for their Moves having a batch of different effects to better control the battle (or hit you hard if you aren't careful). Fairy-types have a similar vibe, though their tricks are more straightforward of hitting decently but its their secondary effect which could start adding up. Eitherway, both Types you want to knock them out asap and a good amount of your party needs to be ready to hit hard as soon as they're sent out. Types that they're weak to shouldn't be too hard to get early either as Pokemon or just moves.
Ghost: Ghost is super tricky because of their Normal immunity. If anything that would be the "gimmick" the a First Ghost-type Gym would rely on. Can't use that high PP Tackle, Quick Attack, and Swift you've been using to preserve use of your stronger moves. Nah, gotta start using the special Type moves (not like the early ones available don't also have high PP making preserving them a moot point). Not only that but Ghost can be a mixed bag: some can hit hard, some are very defensive, others are good status inflictors. So Ghost would be an interesting Type to see how a player can quickly adapt to having some Move options limited and different strategies Ghosts can have to one another. That all said, not only do a lot of Pokemon learn Bite, but the Starters STAB moves would very likely pull them through (though they gotta beware of Moves like Grudge, Spite, and Destiny Bond... as well as Curse and Confuse Ray; maybe a lesson Ghost can teach is there's no shame in switching out).
Dragon & Steel: Why not make the first challenge a player has to do is climb over a big ol' wall. Problem here is of course you don't want to bore the player, so depending on which they use they're going have to include gaps in the armor. In addition, while not a problem for Dragon-types, Steel-types aren't exactly attackers and they're support moves aren't much of anything either so they would be relying on another Type or coverage moves to keep the battle interesting.
Ice: While they'll probably keep Freezing away from a first Gym, Ice is a good Type to teach about Weather. As the player wouldn't have Ice-types all of their Pokemon would receive hail damage and the boost to Ice-type Moves Power would help mitigate the Fire and Water resistances to them. There's also the Abilities which activate in Hail, Aurora Veil for added defense in Hail, Haze & Mist to control stat changes, & Icy Wind to lower Speed. So you don't really need to touch the freezing mechanic for Ice-types to have a gimmick, and Ice-types are a fragile Type so you don't really need to work too hard to get a Pokemon or move to counter them. They would make for a pretty good if not quick surprise First Gym.