Metagame premise: The move in your first slot applies the
secondary effects of its Max and G-Max versions.

Charizard @ Choice Specs
Ability: Solar Power
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
IVs: 0 Atk
- Flamethrower
- Weather Ball
- Overheat
- Focus Blast
Here is a pretty standard Charizard. It has Flamethrower in the first moveslot. This Flamethrower still has 90 BP, 100 Accuracy, and a 10% chance to burn, but additionally will set Sun if hit successfully like Max Flare would.

Charizard @ Choice Specs
Ability: Solar Power
Gigantamax: Yes
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
IVs: 0 Atk
- Flamethrower
- Weather Ball
- Overheat
- Focus Blast
Here is also Charizard, but now with G-Max. So instead of Flamethrower setting Sun, all non-Fire types take 1/6th damage for 4 turns.
This Metagame would be what MnM was for Gen 6 and what Ultimate Z was for Gen 7, but with Gen 8.
Potential bans and threats:

The effects of Max-Ooze, Max-Knuckle, and Max-Airstream are all extremely overpowered when they were limited to 3 turns, so these moves would definitely need to be restricted.

Gengar has access to G-Max Terror. G-Max Terror will trap any opponent that isn't Normal or immune to Trapping like Ghosts (which are weak to Ghost moves).

Probably one of the best Pokemon in the entire Metagame with its ability Defiant, Sucker Punch, Steel Spike, and Dark/Steel typing. It can punish Flutterby U-turns, Strike Facade/Body Slams/Rapid Spin give it a defiant boost, Wyrmwind Break Swipe/Scale Shot/Clanging Scale are resisted and boost Defiant, Phantasm Shadow Sneaks/Poltergeist and Darkness Dark Pulses similarly are resisted too. A new hazard and easier aurora veil also means Bisharp can switch into more Defogs in this meta.

Raichu-A becomes one of the strongest Electric types in the tier thanks to Max Lightning Rising Voltage and Volt Switch. It can set up the Terrain it abuses by using Rising Voltage or have one of many Volt Switch users set terrain while it switches in to be immediately threatening. If Tapu Lele or Rillaboom switch into Raichu’s Rising Voltage, they lose their terrains, and Raichu is naturally faster than Lele too. Raichu-A also gains access to Grass Knot and STAB Psychic to deal with Ground types that otherwise block Rising Voltage.

Snorlax having ways to restore its own berry while attacking, which like Alcremie, would be it incredibly hard to kill

Pikachu may seem weak, but every electric move paralyzes the target, and that might not be competitive.

With regular Volt Switch, you can set terrain with any Pokemon that learns that move, so Hawlucha and Regieleki have easy access to Electric Terrain for Unburden and Rising Voltage Spam (and they can set it themselves too).

Like with Hawlucha, Swift Swimmers and Sand Rushers can set their own weathers, with Swift Swim also having Flip Turn support too. Barraskewda and Excadrill being the biggest threats.

Like Pikachu, but can poison the opponent instead, which is less more competitive tbh but is also on a much better Pokemon.

Like with G-Max Terror, these guys can trap the foe with their STAB moves on moves much stronger than Fire Spin and Sand Tomb

G-Max Befuddle is also potentially uncompetitive strategy I forgot to mention as it has 3 different status effects that also happen to be attatched to a Pivot Move in U-turn. Butterfree is the weakest between Pikachu and Toxtricity, but a 33% chance to Sleep while using U-turn could be strong (or not since Tapu Koko exists and it can only pull this off against something like Blissey or Choice Locked Pokemon).

Blastoise got Shell Smash in Gen 8, and while still walled by Toxapex, Ferrothorn doesn’t like taking 1/6th health every turn for not being part Water.
Venusaur is absolutely a monster in sun, especially with G-Max Vine Lash heavily damaging Non-grass types, while also packing Weather Ball and Sludge Bomb for Grass types.
Charizard can deal with Tyranitar better by either removing the Sand it sets while it switches in, or making sure Tyranitar takes heavy chip damage on its way in, but still has limited options for Heatran.

While Kingler is often overlooked even with its G-Max form, Foam Burst is actually pretty threatening on it. A Life Orb Sheer Force Liquidation hits really hard, and when you can lower the switch-in’s speed by 2 stages, something like Zerarora no longer outspeeds Kingler, which is actually insane.
Same thing with Obstagoon’s Facade. Any non-Ghosts and the likes of Regieleki become immediately slower switching in, and the Ghosts don’t like Guts boosted Knock Offs either.
Questions for the community:
Should G-Max Pokemon be tier separately from their regular counterparts?
How should problematic Max moves be dealt with?
Would another form of restriction be better than the effects applying to the first move slot?
Should status moves gain the effects of Max moves, or just be restricted?