Gmax-gengar looks like an exaggerated version M-Gengar, and gives ghost moves shadow tag, it's M ability.

G-Charizard looks like an exaggerated merging of its M versions, and sets up a Not!weather that does does extra damage, a merging of toughclaws and drought.

Assuming this pattern continues, and we eventually get a G-Max of every Megamon with a similar if altered toolset, what do you expect to see?

>gardevoir, tyrannitar, abomasnow, gallade, glalie, steelix, gyarados, and lucario have no g-max
yet