Hi! I also use Trisquel with an AMD/ATI GPU. I don't know any exciting tricks concerning that. It doesn't seem like AMD GPUs will run in full capacity in a completely free system any time soon. AMD doesn't allow it. See: https://trisquel.info/en/forum/3d-and-free-software-trisquel#comment-4170
I heard that almost all Intel GPUs are fully capable in a free systems such as Trisquel. I haven't played many computer games since I started using only free software, but one game I've played and really liked is 2D strategy called “The Battle for Wesnoth”. You can find it in the repository. I also found out that you can do 3D modelling with Blender without hardware 3D acceleration, bearing only little inconveniences.
