The retainer plates on the gas tube are spot welded. You cannot "drive them off" without damaging the tube.
Place the gas tube in a vise with the front end sticking straight up. The vise jaws should hold the flat areas on the sides of the rear portion of the gas tube (the end that goes in the rear sight block). Don't overtighten the vise. You just need enough pressure to hold the tube securely. You can also use vise grips, large pliers, or an adjustable wrench to hold the flats. The wrench method requires some acrobatic maneuvering to hold the tube and wrench. Once you figure it out, it's not too tough.
The handguard is U-shaped with a ledge on the front and back. This ledge holds the guard in the retainers of the gas tube. Wrap a shop rag around the handguard and twist
the actual wood or plastic guard 180 degrees around the gas tube. You want to rotate the guard so the gap is to the top of the tube. These can be tight, so the rag will help give you a better grip. Once the guard is rotated, you should be able to work the guard off the tube.
There's a tutorial on the Romanian Kalishnikov site that's pretty decent. He has some other picture tutorials that are useful, too.