It should be a stand alone code just as it is now.
Historically it has been the plumbers that where trained to installed gas lines. Today even FL requires mechanical contractors to know the fuel gas code as part of their exam.
After 10 years or so of ONLY accepting our local test, about five years ago when we started accepting the ICC Master Mechanical, AND the ICC Master Gas Pipe Fitter, we had more contractors opting to go with ICC's test. Ours was not a trick test, the answers were word for word, until they had to start calculating. I obviously I am comfortable either way.