you can absolutely sell your code and still license it under GPL. Just, if someone buys it, and it is really GPL licensed, then they must be allowed to redistribute it without further restrictions. So any third person can then get it for free. So the only reason to buy it from you would be "because it's the sane/right thing to do" which makes it look very much like a donation...

IIRC there was a project that tried a "we give it to you as GPL, but if you redistribute it, we'll not sell you future versions" kind of deal. I want to say some kernel patch set? At the time there was quite a bit of debate about whether that's okay