The GPLv2 is not a contract, it is a revocable license.

Here is a paper explaining what the GPL is and is not: http://illinoisjltp.com/journal/wp-content/uploads/2013/10/kumar.pdf

(With full citations). (PDF attached)

Page 12 starts the relevant discussion. Page 16 begins the explanation of all the ways the GPL is not a contract.

Later there is a short gloss of state law promissory estopple doctrines. Remember: in the case of the linux kernel it, unlike other projects, omitted the "or any later version" codicil, and is only under version 2 of the GPL, which makes no promise of irrevocability by grantor.

(Note: The SFConservancy recently chose to publish a "correction" that conflates clauses, within version 2 of the GPL, [that clarify that if a licensee's license is revoked by operation of the license for a violation of the terms, that sub-licensees licenses are not-in-turn automatically revoked] - [with an inexistent irrevocability doctrine within the text of the GPLv2]) (Additionally: Clause 0 of GPLv2 specifically defines the "you" in said clauses as referring to the licensee (not the grantor); the SFConservancy's conflation is shown to be ever more disingenuous)

The Linux Kernel License grant: Is Not: a contract. [No breach of contract damages vs grantor if rescinded] Is: a bare license akin to a property license. And: There is no "irrevocable by grantor" promise in v2. [No promissory estopple defense] .: Can be rescinded at will.