Yeah, it is. Sorry, I should have mentioned.
Thanks. Between the two sets (SD / Waterfall / Ice Punch / Earthquake or SD / Agility / Waterfall / Return), is one clearly better, or can I safely just choose based on personal appeal (I like the first one a bit more, but don't want to short-change myself)? Also, I notice that none of the UU sets max out speed, but since they all had either Dragon Dance or Aqua Jet, I figured I should compensate with 252 Spe EVs. However, I realize that speed EVs are determined with the aim of out-speeding specific threats, and I don't know if 252 Spe EVs beats anything that a lower number wouldn't also (in which case I'd dump the extra EVs into HP). I can use the calculator on Pokemon Showdown to try and figure it out myself, but I don't have a perfect sense yet of what all I should bother trying to out-speed where I could, and I figure someone here might know the answer off the top of their head.
As for Jolly vs Adamant, what would you recommend if I told you I'd be willing to just build a team to suit it either way? Is there more potential in one of them with that amount of flexibility, or is it still kind of apples and oranges, pick your flavor?
Apologies if this question no longer qualifies as "simple".
For Jolly vs Adamant, 252 Speed Jolly beats out neutral-natured 252 Speed Moltres and Kangaskahn by 1 point (280 vs 279), so definitely worth full investment if going Jolly. Adamant only needs 232 Speed EVs to beat out the next notable thing (Adamant Absol, which has 249 Speed), giving you an extra 20 to toss into bulk. As for which is better, it depends on the set and what you're aiming to do with it. Adamant will always make a difference, while Jolly will only make an impact against certain match-ups. Those match-ups are pretty important, though.
Take Modest LO Moltres, for instance. Adamant deals more damage by ~5%, which does make a difference overall (figuring in its own recoil, Moltres will usually get KOed by Adamant Waterfall but rarely KOed by Jolly). If you're using LO yourself, Adamant always KOes while Jolly misses out ~6% of the time. That Speed difference, however, determines whether Moltres gets to dish out 79.4 - 93.5% to you (or straight KO with a crit) first. If you're running Adamant and have taken SR damage, LO recoil, etc., that can take you out before you even get a chance to hit it. Jolly on the other hand can pull off the KO if you have SR up, or Moltres has taken some chip damage from LO / stray resisted attacks. Speed is the most important factor if that Moltres has taken some damage already, you've already set up, you crit, or you just want to heavily damage (and possibly KO) it before going down.
Or Qwilfish for another example. If you're up against SD or RD Qwil, EQ's gonna KO regardless of Adamant or Jolly, but Qwil can take Adamant down with Explosion. Against more defensive sets, the extra damage is better since you'll outspeed regardless (and it can even guarantee KOs vs. certain sets at full health).
Really, all the threats that I mentioned before are offensive. You can generally beat them if you're faster, but take heavy damage / possibly get KOed first if you aren't. If you want to fare better against those specific Pokemon, it's worth going Jolly. If you think they won't be issues, go for Adamant.
Best set depends on team needs and what Gatr is ideally accomplishing. The most effective set overall might be more restrictive to build around. I'd recommend trying out the different sets on Pokemon Showdown before taking the time to RNG and train it, as that way you'll have a good idea of what set best suits your style. As for what the sets accomplish:
The straight SD set makes for the most effective breaker of the bunch. In other words, it's the best at busting up walls and dealing raw damage to ease the path for a sweeper / cleaner to finish things up. Against slower teams, it's by far the scariest set. Faster teams will generally have an easier time taking it down, in turn making them harder to switch into and set up against. That said, they'll have a tough time switching into you as well.
The SD + Agility set is scarier than the above for offensive teams, while defensive teams have an easier time covering it (though it can still hole-punch well and has a possibility of sweeping). You're essentially trading coverage for versatility. This set aims to threaten any team, making it an interesting sweeper or glue mon, but it'll need something else to chip things up first before doing its best work.
A third possibility would be Scarf, which can be pretty versatile in itself. Jolly is still cool then, letting you outspeed anything from Jolly Swellow to Modest Scarf Moltres. That said, you might want the extra power from Adamant, since you have no way of boosting. I'd use the 3 attacks from that SD set, but you actually have several options for that last slot. Crunch, Superpower, or Sleep Talk would probably be the best picks, but Gatr has a surprisingly large movepool (Rock Slide, Outrage, Shadow Claw, Low Kick, Return... not too shabby when it comes to surprise possibilities~) This set aims for hit-and-run maneuvers and revenge killing until it gets a chance to clean up a battered team.
Also, yeah, I think I went past "simple answer", haha. Sorry if that's tl;dr or incoherent, I'm a bit baked atm. Feel free to shoot me a PM if you want to discuss this more, though~