Do Unix systems and other similar systems use the GPU for the startup splash/loading screen (when there is one)?

I was wondering if Unix systems use the GPU for the startup splash/loading screen because I've been having some trouble with an overheating Mac with graphics issues. Unix-type systems (such as MacOS 10.6, 10.10 and different versions of Ubuntu) show the splash screen, but never actually boot into the GUI (typically just a plain black/blue/white screen after the startup splash). Windows, however, starts up (I assume this is what's happening as I can hear hard drive activity) and only shows a black screen (no splash or loading screen). This just made me curious as I have a cursed 2008 ATI iMac. I plan later to try reapplying thermal paste to see if that does any good, and then try a reflow (I know this'll only be a very temporary solution but I just want to see if anything will work), but if all else fails, it'll probably go into the bin.