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

From my one course on Business Law over 30 years ago (so my memory may not be right), "Consideration" and acceptance of an offer doesn't have to be goods. Example given, as I recall: "If you get an A on your XYZ test, I'll buy you ice cream." That's the offer. Acceptance can be demonstrated by getting an "A" on XYZ test. Consideration is, on one side, the fact that the other person got the A (on the other side, they get ice cream). From my perspective, the receiver abiding by the GPL could be considered acceptance, which binds the giver to the terms of the GPL for providing source code.

IANAL, though, having only had that one course. For legal advice, consult an attorney.