I'll be the first to say that some of these buggers are staked tight as all get-out, and honestly, some might require a gunsmith, or at least some significant creativity to get them out.
There appear to be three staking methods used to fix these screws:
1) A single linear punch that runs across the outer portion of the screw head and then onto the gas block where they meet;
2) A single hand punch(...?...) on the screw head that angles outward towards the gas glock;
3) A little non-descript (sometimes nearly invisible at a casual glance) "stamped" area that distorts the edges of the screw head and the recessed area along the gas block where they meet
The single hand punch version (2) is the easiest to remove, as you can just take a punch and drive the outer staked edge of the screw back towards the center. The other two seem to require a progressive series of ever more "forceful" approachs.
Here's a few pointers and ideas for the stubborn ones:
1) Soak the screws with a quality penetrating oil for a few hours. Using a machinists or cutting oil is the best choice, but most any quality penetrant should help.
2) Make sure you use a high-quality, properly sized hex wrench that will take a high amount of torque without stipping either the screw head or the end of the tool. The "T-bar" handled versions in chromoly or good hardended steel seem to work very well for this.
Remember, gunsmithing is NOT the time to use a .99 cent el cheapo tool from the bargin bin at Wal-Mart, because if you strip out the heads of the screw, you could be in for some time/$$$ consuming work to get that screw out.
3) After you have soaked the screws, go ahead and see if you can break the screw loose with the hex wrench. If not, try attaching a pair of vice grips or other type of extension to the hex wrench to gain some additional leverage.
4) If the screw still won't budge, remove the stock, wipe off any excess oil, grab a blow torch, and then heat the gas block area and screws until they are nice and hot. Not glowing red or anything like that...just nice and hot. Soak it with oil again, let it sit for a few minutes, then give it another try with the wrench.
(The "oil-->heat--->whack it with a B.F.H. method" is a tried and true nut/bolt/screw removal technology for guns, cars, tractors, earth moving equipemnt, and airplanes.)
5) Locate exactly where the staked area is, and using a punch and hammer make a series of taps at that position inward toward the center of the screw. You don't really need to deform the screw...just nice solid blows that will cause enough "minor damage" to dislodge the staked area.
6) A more serious option is to take a tiny drill bit...say a 1/32" or 1/64" if you have one...and very precisely drill down along the very outside edge of the screwhead near the staked area. Only drill a down a very short distance...about 1/16 to 3/32" of an inch will be plenty. Now, take the punch again, and this time drive the outer edge of the screw inwards towards the center.
7) If you can find a good screw extractor that you can get to bite into the screw, you can often get the staking to break free. I've heard that a torx (t-20) bit can also be driven into the head recess, although I have not tried verifed this method.
This should about cover getting the screws out...getting a good fit on the block is entirely up to you and your torque wrench.