Blowing out moisture from gas line to furnace
Gas pipe CAN accumulate "junk" inside, oil impurites, etc, but water, doubtful. If it did it would freeze up the meter and regulators.
"Several hours." You need to post a photo or two and brand / model of the furnace. Much more likely something wrong, right there
The pipe sitting there "for a year" or whatever should still have gas in it.......there's no place for it to go, LOL Sort of like air pressure in a tire