The GSOC projects are not trivial ones, AFAIK, so the target is a bit different from yours. I also think that the project you mention are complex, with multiple reasons for the high barrier to entry.

If you like OSs, and you're looking for a low barrier to entry, my guess is that you could try working on Wine. I remember seeing fixes that are the addition of a constant or so (but I have no experience in the field, so I'm not sure). This could give you some basics, and pave your way to contribute to ReactOS.