I know this was necro'd but it is an interesting thread to me. I was thinking about making one about best GOG games to potentially run on like a $300-500 office computer.
The most obvious candidates are probably DOSbox games that aren't 3D intensive. Older 2D indies also, from when indies started to become popular in the Steam era (World of Goo, Braid, Aquaria, etc.)
I got Flatout 1 running on an old Win7 office machine for some friends not long ago.
The main non-GOG free games that come to mind are Blupi games (especially Speedy Blupi 1 and 2), and Hamumu Software games, namely Spooky Castle, Supreme With Cheese, and the Loonyland games (I had trouble running them on older post-XP machines but Spooky Castle at least ran fine on my Win10 machine).
http://blupi.org/ ,
https://hamumu.itch.io/