Would take some time before it would be playable. Stephen for example would've had to get used to it (if he was to convert to new version) and make completely new content etc.
There won't be a Garry's Mod 2. New major releases of Garry's Mod will be called 14, since we are already on Garry's Mod 13.
I'll be on point here: Do I think Garry and its dev team port the old Garry's mod over to Source²? Yes. Since we have high hopes of CS:GO being ported over to Source² (Aswell as Dota 2) I wouldn't see Garry's Mod take advantage aswell of Source². Source is pretty much done trough out the years, its basically a tower of rocks that gets fixed every time with duct tapes (updates representing the duct tapes) so it won't fall over and break. Every update that gets released just brought us more and more problems, CS:GO's memory leak should still not be fixed aswell as performance problems. Honestly, can't wait what Source² will bring us. This one is for sure: it will increase productivity.
Garry doesn't give a single fuck about gmod, it took him 3 months to fix wilox's request_file exploit. He will never ever get off his ass to make something which won't make him money unless he makes people pay for it