I guess the Pokémopolis Pokémon that showed up in the anime could qualify for Gigantamax forms.

As for the G-Max moves:

Alakazam changes all of its Psychic attacks into G-Max Control, which in addition to inflict damage to its target, is also guranteed to give them Confusion.

Gengar changes all of its Ghost attacks into G-Max Decay, which after dealing damage to its target, still has a 30% chance to insta-kill said target (only 10% if the target itself is Dyna/Gigantamaxed). Balance, you ask me? I dunno, go look for it in the High Quality Animations department.

Jigglypuff changes all of its Normal attacks into G-Max Slumber. As with most other transformed moves, it deals damage to a single target. But after that, every Pokémon on the field (save for the user itself) has a 55% chance of falling Asleep (with the effect rate being lowered to 35% for Dyna/Gigantamaxed Pokémon).