You’d probably need some RAM for that. Otherwise, there isn’t exactly any chance of that happening. The game of Gmod loading takes up enough of the resources, I don’t think a simple minigame would fit in.
Play in window, open tetris?
Just go in Windows mode and do whatever.
How about you take a brake from your computer and go outside.
This is what gameboys/phones/ipods/books/Game consoles and other devices besides the PC are for, or simply go do something else non electronic related.