Yes, I have installed NixOS on my GPD Pocket 2, which works OK. Have not encountered issues. The optical mouse actually works quite well and my hands can operate it with thumbs, while walking around or lying in bed, as well as more typical typing while stationary.

I would advise against getting a GPD Pocket 3. It's basically the size of a normal netbook and defeats the "pocket" monicker imho.

Depending on your distro you may have to set up a configuration that rotates the screen and/or digitizer to the right orientation.

I have the GPD Win 1 and GPD Win 2. Ubuntu MATE fully supports the GPD pocket PCs. Most distros will run fine on them because they're Intel SoCs, but MATE had the earliest support for stuff like getting screen rotation right and bundling all of the more obscure drivers