"Gas" or plumbing pipe are the same. Most people making this kind of comparison are thinking (natural) gas lines or some water lines. It usually comes in two types, black for natural gas and galvanized for water, although galvanized has fallen out of favor to other tubing materials. "Tubing" has a different diameter and wall thickness from pipe.

However "gas pipe" isn't in use in bicycle manufacturing. "Gas" pipe is schedule 40 pipe which has a wall thickness of 0.14" (3.5mm). Even a heavy bike like the LHT uses frame tubes that have a 0.8mm (0.031") wall thickness at the butts on the end of the tubing.