You can clamp the gas tube in your vise gently, with out crushing it.  Then you can
rotate it, to break it loose.  So it turns in the FSB, then it's just a matter of wiggling it out.  The secret is to get it to turn in the FSB.  You could also use vise grips.

Weazer

rag and pair of pliers, twist as state above. i have even had luck holding the tube with some pliers or vise grips and tapping towards the reciever with a hammer

Stuck Gas Tube Thread

Quoted: Well..I snapped a pair of vise grips on it this morning gave it a twist and it came right out. Thanks for the help folks. Now..any secrets for getting the barrrel nut off...

Make sure you have a good barrel wrench and the upper in a vice, you should have a vice block to do it but I dont think you do.... so if really want to go for it then heavily pad you vice and keep the B/BC in the receiver and try turning the nut, if it dosent wnat to move or you feel like you may be causing damage then stop and either get the right tools or pro help......but I would say get the tools! its a lot more rewarding!

Ya better off... I did it for a while with out a vice block and luckily never had any problems, but life is so much easier now with the right tools

I had the same problem.  Bastard wouldn't come out after knocking out the roll pin.  So I checked on ARFCOM and used a pair of vise grips and a hammer.  Actually, I twisted the tube back and forth with my fingers first to loosen it as much as I could.  Then I took the vise grips, clamped them on pretty tight and gently knock them with a hammer.  Voila, the gas tube budged and came out.  The copious amounts of WD40 I sprayed on there probably helped as well.    Gas tube was undamaged.  

that  gas tube wrench from bushmaster really works well, but Ive had issues with a few gas tubes that were stuck due to carbon build up. Twisting it with the wrench will help most cases and squirting some breakfree in there helps a lot with the stubborn ones. I have had one that wouldnt budge, and finally decided to "break" it loose by bending the thing (plan was to install a spare I had laying around).  The thing finally came out alright, but the tip broke off and got stuck in the Front Sight Block.  I just sent the whole barrel back to Bushmaster and they did the work for free (since it was a bushmaster barrel).  came back with the FSB drilled out and I just put it back on the receiver.  Out of a dozen rifles I have assembled and the many more I have taken apart and re-assembled this was the worst case scenerio.

Now I usually squirt a bunch of breakfree in the front sight base before installing the gas tubes. I dunno if it helps or not, but it does seem a lot easier to remove them when I take them apart later for routine maintenance.