To add to this: here's a source confirming the FSF are ok with non-Free game assets (and other non-functional data). [0] They single out documentation specifically, saying that documentation should be Free despite that it's non-functional. [1] See also [2].

[0] https://www.gnu.org/distros/free-system-distribution-guideli... (under Non-functional Data)

[1] https://www.gnu.org/philosophy/free-doc.en.html

[2] https://www.gnu.org/philosophy/essays-and-articles.html#exte...