tfishell: I think it's truer of "AA" (and lower) devs and pubs than AAA's though.
What is the meaning of AA or AAA? According to my knowledge this is a school-rating for some schools in the US. How comes we are linking it to a certain quality or quantity of a game?
Regarding the costs involved, we can usually only guess... because the exact costs are rarely revealed.
It would be more accurate just trying to get a estimate of "the amount of devs involved" for a certain game and in general a bigger playtime will mean it is a rather big (and rather pricy) title.
Usually what we may miss on GoG, or any of the "smaller platforms", are "huge franchises" which is a certain game or series with special popularity and generally notorious for being able to run very well on "huge" or monopolized platforms. To me, any platform able to become a share of over 50% (of something comparable, for example games for the PC market), i do consider critical.
Hades for example... it comes from studio that was previously (until a certain point) barely known at all... and then it got famous and infamous (both at once) over time and as soon as Hades was released... they stopped supporting GoG, because most likely they had become cocky... and not even the old Epic-sponsors (with exclusivity-deals) are considered important to them anymore... the biggest platforms only.
Being popular does not necessarily mean "a big studio": For example "Grim Dawn", is actually coming from a small studio (smaller than Supergiant) but to me it is on par with any game on its genre. Of course it was rated high for a reason, but big is not always better. I still enjoy to see more of the "big studio-games" on GoG, although... it does not automatically set the quality of a game.