I'm switching barrels on my 20" and putting on a lightweight. The roll pin is out, but the gas tube isn't sliding out. how much pressure should I need to get it out?

Depends on how tight it is. If you have a smooth jawed vise you can use it. Smooth jawed or leather padded Vise Grips work. Brownells has a nice tool for this too. They usually stick because of a carbon build up or burring between the gas tube and front sight base.

you can use [b]gentle[/b] pressure with some pliers. or use a special tool [url=http://www.brownells.com/aspx/NS/store/ProductDetail.aspx?p=1851&title=AR-15%2fM16+TOOLS&s=7188#7188]GAS TUBE WRENCH[/url]

That gas tube wrench works great, and I have used it to save several otherwise stuck tubes.  But you have to consider whether it's worth the $24.95 plus shipping just to save one gas tube. After all, they are pretty cheap.

There have been times when I just taken a pair of Vise-Grips and severly crushed the gas tube up near the front sight base, then tapped on the pliers with a hammer while twisting to work the gas tube out.

Of course, after this you need to replace the gas tube with a new one. But like I said, they're cheap.  Plus, now you have a gas tube you can cut into a 2"-3" stub and use as a handy barrel nut alignment tool.  Slip it into the bolt carrier key and apply forward pressure on the bolt carrier in the receiver.  The tube will stick out the receiver, keeping the circlip, weld spring, and delta ring indexed, and pop right through the barrel nut when the next notch lines up.  (No torque wrench needed.)  It turns barrel installations into a ten minute job.