Is the contract restricted to the buyers, or to the "any 3rd party" of the GPL?

Your example is confusing because it imagines a product X and then asks where X provided consideration. Products don't accept contracts and so needn't provide consideration, people do that, and so I suggest that you mean to ask what consideration A has received from B ?

English Law has a *long* history of finding some consideration where a lay person can't immediately see anything, so I don't expect that to be an obstacle here. Go read about Peppercorn rents for an example of this. Today it's even not legal to demand payment of the peppercorn because the whole point is that it's a legal fiction, you don't get an actual peppercorn. Lawyers were apparently not uncomfortable with the idea of "consideration" which doesn't exist and will never in reality be received by the other party.

So in this scenario my guess is that if obliged to find one lawyers will say something like B might improve this source code, and A can benefit from that, so that's a consideration. Did they improve it? Nobody cares, in principle they could. Did A benefit? Nobody cares, in principle they could.