As far as I understand AppleCare, that components are under warranty just as part of that machine.
For genuine Apple parts: for example if you bought Apple Radeon HD 5770 as upgrade kit, you cannot get AppleCare for it (3 years warranty), just 1 year limited warranty.
So, of course you can sell your GPU, but it is out of warranty outside original machine.
My advice is, keep it as spare.