If it was just an engine then the GTA 3 “port” is i believe legal. Just another company abusing it’s power.
As for GTA 5 VR, I know they can’t make any money of their work or they’d have something worse than a just a mere DMCA ( Nintendo made someone pay it half his income for life ) but I know the most devs are mostly doing it for the challenge and more importantly out of love for the game. As an exemple, here’s someone porting portal to the N64