Never done it before, and I need to tighten my barrel nut another notch.

Who built the upper?

The gas tube is held at the front by a roll pin through the gas tube bracket of the front sight base.

Driving out that roll pin frees the gas tube and you can then slide it to the rear until it clears the rear of the front sight base.

Gently pull the free end of the gas tube to one side of the FSB (it helps to rotate the tube so that the bend in it is to the outside) and pull the tube forward until the rear end is clear of the receiver.

[speeling]

Um-m-m-m-m........not Colt.

If you find the gas tube hard to remove from the front sight block because of carbon build-up try this: clamp the gas tube a few inches from the front sight with a small pair of vise grips.  (Just enough to get a hold on it, not enough to cause the tube to dent.  Start loose and tighten the vise grips a little at a time.)  When you have a firm hold on the tube use a small hammer to tap on the vise grips (near the gas tube) as you hold them.  This worked greast for me when I installed a free float tube.