USA has numerous different models, it changes with each state but what is fairly consistent if the trade test, usually related to theory based state building laws.
Problem with using the apprenticeship as the measure is you say after it you know everything, you don't, you know enough to work, but then you need experience of working without supervision, hence my suggestion of a further year.
The regs don't say if you don't get paid they cannot touch you, they defiantly can, the regs state you must be competent, so if your not qualified, registered or not I don't see how you can prove that
The four year apprenticeship already includes gas, your suggestion of 7 years wouldn't work, you can get gas qualified by doing your ACS, nobody would bother. ACS is seen as a short cut and one of the bigger issues than a two year college course which you already sign to say doesn't lead to competence