1997 GMC Jimmy head gasket?

Hello! My car has been leaking antifreeze for a few months now, I have been filling my radiator every couple of days with antifreeze. And the overflow has only emptied on me once. When I open up the radiator to fill it, it almost looks like there is oil mixed with the fluid. It is only a tiny bit- hardly even noticeable, but it's there. (It's not in the overflow, however.) I've heard from others it may be my thermostat clogging, but for the most part I've heard it's my head gasket. No smoke (or steam) has showed coming from my tailpipe, and the leaks seem to happen in huge pools instead of little by little- almost as if there's air in the cooling system that ends up "burping" out. What do you think it sounds like? And at this point (between the year and the mileage) do you think it would be better to fix the problem or try to find a new car? How expensive do you think it would be to fix the problem? Is it possible to do it yourself, and how hard is it? Thanks for listening, I'm worried about my baby.