Tip for Linux : you can play it through Steam's Proton by doing Games->Add a Non-Steam Game to My Library and then browse to select Mobile Suit Baba.exe.
If you use flatpak steam, make sure to give Steam access to where the executable is located (Steam Flatpak Wiki).
However, I'm using Fedora, and the game starts in a tiny little window and the fullscreen option is broken (the content scales up but not the window).