In such a case promissory estoppel would be a very good defense. Distributing your work under the GPL is a strong indication that you intend others to build on it, and their reliance on this is reasonable.
In which case you've essentially got a contract, with promissory estoppel serving as a substitute for consideration.
It's interesting that if one searches, in either law textbooks or in the case law, there just isn't much that makes a distinction between licenses and contracts outside of real property law, where they are usually talking about bare licenses. In almost all other contexts, the terms are largely used interchangeably, although it seems license is more likely to be used when it is a one to many arrangement (e.g., one party is making essentially the same contract with numerous other parties) and contract is more likely when it is one to one.
