There's also a legal theory that the GPL is, or can be depending on the circumstance of a particulsr case, a contract license rather than a gratuitous license [0], but even in that case the contract offer would be revocable, and the GPL itself only irrevocable with respect to licensees with whom a contract was formed prior to the offer being revoked (because FOSS licenses are sublicensable, that licensee could offer sublicenses to the original work, but might not, e.g., if they chose not to do any of the acts which required such an offer.)

[0] which may not actually save the day, either, since some courts have held that contract licenses are revocable, but that revocation in violation of the terms of the contract makes available damaged for breach of contract rather than compulsory extension of the license.