GNU is not Marxist. Like, flatly, not at all. To say so is just a mistake or a lie.
GPL says literally nothing at all about workers, employment, capital, or revolution. It makes no requirements addressing the rights of workers who develop software on behalf of capitalist corporations. GNU does not advocate for workers owning the means of production, or against capitalist corporations alienating those workers from the product of their labor.
The GPL doesn’t say “If you employ workers to work on this code, those workers rather than their employer must own the product of their labor.”
Rather, it is entirely focused on granting rights to software purchasers and users.
This makes sense if you know movement history. The inspiration for the FSF and GNU originated with RMS’s experience as a customer denied access to improve printer software that his MIT lab bought and used.