Lapras Hatterene Charizard and possibly Grimmsnarl are the only gmax pokemon who seem to benifite more than just having dynamax. The vast majority of the dex gains benifite from dynamax's main core mechanics
>double hp
>unstopabble, protect-piercing attacks
>passives from attacks
In fact a bunch of Gmax attacks dont assist but hinder the basic Dmax mechanics. For example, Orbeetle can set up Psychic terrain with his dmax Psychic moves or can set up Gravity with his Gmax. I'd argue the terrain is typically more significant for his personal use, independant of the team synergy. Many Pokemon have this occur in leu of the boost or field effect that would naturally have a s a dynamax. Now the argument of gmax Copperajah or Corvaknight could be extended because as a doubles player I find much less use for them over airstream or steelspike boosts.
My point is dynamaxing has done one thing right; the vast majority of advantages stem from the default dmax boost, which is the aspect available to all pokemon (contextually as movesets and typing vary), so it's not nearly as impactful to have a gmax unless it specifically aids that species.
TLDR: I don't think measuring the merit of a gmax and mega use the same logic and therefor arent a very helpful comparison when discussing mechanics.