It was a live issue in the Artifex case. The parties ultimately settled so we don't have a final answer, but the district court was going along with the contract theory. The availability of specific performance remains an open question too. But if you can in fact enforce the GPL as a contract, then it's not a big step to some plaintiff getting specific performance, which is going to turn on case-specific things like the adequacy of monetary damages.

https://www.synopsys.com/blogs/software-security/breach-gpl-... https://www.omm.com/resources/alerts-and-publications/alerts... https://www.natlawreview.com/article/important-open-source-r...