use penetrating fluid and/or solvent, let it soak and then try again. If that still dosent work I have two suggestions, both of which most will not like or agree with but they work.
Replace the gas tube and just pry it out with some pliers (careful not to break it off in the gas block...). I run into the 1 in about 20 rifles in the military that the tube is truly stuck and i just replace it. dosent hurt to replace it anyways cause its all nasty inside...
Take some wire cutters or dikes and gently squeez the gas tube near the gas block, then using the gas block as a fulcrum and the small incesions made into the sides of the gas tube as grip on the tube pry and slide it out and repeat if necessary. This may not work and sometimes it just marrs a long smooth streak along the gas tube...see option 1 (my experience you have about 2-3 chances of the dikes slidng down the tube before it gets to thin to be of use)
As I said about 1 in 20 are stuck like you describe and out of the stuck ones about 1 in 5 I end up not being able to re-use. If you need a picture I can send you one if you do not get what I am describing. If you do not like the marks just stone them off before installing it again and order another cause next time you will not be able to do this without cutting the tube...if you do break your tube off the in the gas block you can drill it you carefully but i reccomend just replacing the gas block...I have only broken off 1 tube and i drilled it out with success but I imagine not all will have as much patience as I when it comes to a 10$ part.
As for the barrel nut use some heat, leverage, and a GOOD wrench that looks like this...
http://media.midwayusa.com/productimages/large/271/271183.jpg
dont use these
http://media.midwayusa.com/productimages/large/762/762854.jpg because the lower number of points of contact with the barrel nut make them prone to stripping the nut...then you have a few hours of fun ahead of you with a grinder, file , and vise grips or something along those lines.
I know it says mil spec but that dosent mean its the best it just means they are the lowest bidder.
