Not popular enough, nor is there anything even remotely standardized about gassers in .300 WM.
NEMO Arms & Falkor Defense make .300 WM ARish factory rifles. They seem cool but they are significantly more expensive than a .300 WM bolt gun.
Probably never . Sucks though a good gas gun will have you questioning from a functional standpoint if bolt guns make any sense at all. Not sure I'd want to be one the first to charter into the territory of managing the pressure of 300wm in a gas gun build though.