How difficult would it be to remove a gas block and install a fixed front sight?  Any special tools?

Factory barrels are drilled and reamed on a jig that holds the barrel and front sight base (FSB) in the proper alignment.  You must be able to drill through the FSB and the barrel at the same time.  If you don't have the FSB exactly orientated on the barrel, your FSB will end up canted to one side and you may not have enough windage adjust in the rear sight to make up for the cant in the sight.  A good sturdy drill press or mill would be recommended.  A proper sized taper reamer is also needed for the taper pins.  Some companies are using straight pins on their FSBs, but you would need a proper sized straight reamer to size those holes also.