Pokemon: Groudon
Type of the Gigantamax Form: Ground/Fire
G-Max Move Name: G-Max Desolating Drought
G-Max Move Type: Fire
G-Max Move Description: Super-effective against Water-type Pokemon. Upon being used, sets up Intense Sunlight that evaporates Water-type moves and resists other weather changing effects (Except Intense Rainfall and Strong Winds), which lasts for five turns (8 with Heat Rock). This move ignores opposing abilities and other effects that would stop it from being used.
Competitive Overview: I hope you guys didn't miss Primal Groudon too much, because here, it's been adapted into a Gigantamax Form! G-Max Groudon is simultaneously better and worse than Primal Groudon. Better, because you don't have to Gigantamax and thus don't gain a ground weakness or a double weakness to water (which is especially dangerous considering its Water-evaporating sunlight is no longer instant), not to mention the fact that its Fire-type Stab can blow through Water-types now like the ever-present thorn in its side that is the big blue whale Kyogre, but worse because its stats are lower, and its water-evaporating sunlight is no longer instant, so one wrong call can put Groudon in a world of hurt, especially if it tries to pull shenanigans against the whale. It still does mostly the same offensive things that Primal Groudon did back in Gens 6/7. Its G-Max Move is an upgraded Max Flare that spawns Primal Groudon's signature weather and ignores effects that would stop it from being used, such as Flash Fire or Primal Kyogre's signature Intense Rainfall, and as stated, the heat of its flames is so great that it evaporates Water-types that would otherwise take them with ease.
Pokemon: Kyogre
Type of the Gigantamax Form: Water/Electric
G-Max Move Name: G-Max Drizzling Downpour
G-Max Move Type: Water
G-Max Move Description: Upon being used, sets up Intense Rainfall that causes Fire-type moves to fizzle out and resists most other weather-changing effects (Except Intense Sunlight and Strong Winds) for five turns (8 with Damp Rock). This move ignores opposing abilities and other effects that would stop it from being used.
Competitive Overview: Just like Groudon, this is Primal Kyogre being adapted into a Gigantamax Form, though somewhat less faithfully than Groudon's Primal Form. Case in point: It's part Electric-type now, whereas Primal Kyogre was a pure Water-type. G-Max Kyogre's Electric-typing lets it blow through a few targets it may have otherwise struggled with. For instance, it has a MUCH easier time dealing with Lugia thanks to its new electric-typing giving it STAB Max Lightning (which, upgraded from the obvious choice, Thunder, will do 46% of Lugia's HP minimum on the first hit (That's through Multiscale, mind you), as well as OHKOing even the bulkiest variants of Yveltal. It also lets it beat out other non-Gigantamax Kyogre and completely eviscerate Arceus-Water, which would otherwise stonewall it, and it loses its weakness to Zekrom's Bolt Strike and Kyurem-Black's Fusion Bolt, as well as resisting Rayquaza's Dragon Ascent. This Electric-type, however, comes with a big drawback:
Kyogre's matchup against Groudon is even MORE risky because it's now weak to Precipice Blades/Earthquake/Max Quake while Gigantamaxed, as well as leaving it vulnerable against the many other Earthquakes shaking the tier. The G-Max Move is basically an upgraded version of Max Geyser that spawns Primal Kyogre's signature weather instead. It also ignores effects that would stop it from being used, so relying on water-cancelling abilities or effects won't work. This includes things such as Storm Drain, Water Absorb and of course Primal Groudon's signature Intense Sunlight.
And yes, the fact that the G-Max moves of Kyogre and Groudon use the names of their normal abilities is completely intentional.