But cannabis is a gateway drug to P.
Just because use of cannabis is very common, so most (and probably close to all) methamphetamine users have smoked cannabis does not mean it's a "gateway" in terms of leading to "harder" drugs.
Alcohol is probably a gateway drug to methamphetamine - based on your definition.
Perhaps "gateway" in terms of someone wanting to "get high" there's no dope around because the cops burned it all, "but hey - the local gang-affiliated dealer has some P, can't be that bad - I'll give it a try". Even if that's rarely the case - it's a damned good argument to legalise cannabis immediately.
My only concern about legalising cannabis is that - like alcohol - it's probably unsafe with adolescents / developing minds - evidence points to that. However those inclined to use it, use it regardless of legal status (the fact that it's illegal probably even attracts some adolescents to use it).
I have seen quoted that use by adolescents in Colorado hasn't increased since legalisation.