Some g-moves have special secondary effect. Like special water-type move (I saw Wailord use) after move will trigger Rain weather effect; Gigantamax Pikachu uses G. Special Move (I think), will Paralyze the target (regarless type, like Ground or Electric), etc... Is not a simple "buff stats", IMO.

The script of Dynamaxing is present in the Pokemon Essentials, Essentials Tutorial section

Better just paste the link here: https://www.pokecommunity.com/showthread.php?t=426770