> The GPL is a license, which is similar to but different than a contract.
"license" is an overlapping category to "contract". Some licenses are also contracts, some licenses are gratuitous. Anything that gives the receiver permission to do something that the issuer has the exclusive right to control (such as, in the GPL, the right to do things which are exclusive to the copyright holder under copyright law) is a license, whether something is a contract depends on whether there is offer, acceptance, mutual consideration, etc.
"license" is an overlapping category to "contract". Some licenses are also contracts, some licenses are gratuitous. Anything that gives the receiver permission to do something that the issuer has the exclusive right to control (such as, in the GPL, the right to do things which are exclusive to the copyright holder under copyright law) is a license, whether something is a contract depends on whether there is offer, acceptance, mutual consideration, etc.