[gunsmith_tony]

The gas tube threads into the gas block and is fixed in place by means of a small pin...visible once the gas regulator nut is backed off. If it's an STG variant, the gas tube is likely shortened and silver soldered to the gas block. Most all other variants will have a full length gas tube as described above.

Unscrew gas nut (from reciever), unscrew gas regulator but from gas block. Drive out gas tube pin, gas tube should unscrew from gas block. Most are not tight.