Since being intoxicated/impaired/under the influence while driving is illegal, regardless of whether the substance is legal or not or if you have a prescription for it or not, I'm not sure why the laws involving intoxication and guns would be any different.
It'd be nice if all such laws could evolve to just punishing
harmful acts regardless of sobriety or lack thereof (due to anything) but I'm not sure why there'd be any changes with the passing of med. marijuana.