Should be two allen set screws on the bottom of the gas block front and back.
So start off by getting two new set screws, Hence pull the one from the block, go to home depot with it, us it in the thread gauge they have there by the bin of screws, and them buy a pack of them (should be a pack of two) the correct thread and lenght. Also, pick up a tube of 272 loctite.
Now before you install the screws back in the gas block, pull the block forward, make a line on the barrel shoulder center line of the gas block in the barrel, then using the center of the gas tube, align the gas block passage correctly to the center of the barrel gas block. On the allen screws and the threads in the block, clean these with actone or finger nail polish remover, start the allen screws in the gas block channels just one thread in, put a drop of 272 on both allen screws threads protruding out of the block, then tighen the allen bolts; again, with the center line of the gas tube to the mark you made on the barrel shoulder center line of gas port block. Once the bolts are tight, q tip with some actone/finger nail polish remover to remove the excess loctite.
And the reason that we loctite the bolts this way (very end of the bolts dry), this prevents loctite from getting in between the block and barrel, making getting the block off down the road a PITA instead.
Now once the block has been installed correctly, pull the bolt off the carrier and using only the carrier with gas key, dry fit it into the upper receiver. The gas tube will enter the carrier key when the front of the carrier is about 1" from touching the barrel extension, and if there is any binding of the gas tube to key as they mate up, the gas tube will need to be tweaked to index it correctly. The best way to tweak the key is to do so over the barrel, but you can tweak the end of the tube in cases where you can get the float tube off to tweak it from the middle instead.
And lastly, do not index rotate the gas block on the barrel when you setting the gas block to get the tube correctly indexed with the carrier key. The gas block gas passage has to be centered on the gas port to not cause problem with gas blockage, then the gas tube is tweaked afterwards to get it indexed with the carrier key.
As for if you need to remove the gas block down the line, using a old allen wrench, insert it into the allen head set screws one at a time while you glow the allen wrench with a torque to transfer heat to the allen bolts to break the loctite bond (will smell a sweet smell when the loctite bond breaks down), then use a new wrench to remove the set screws. If you don't break the bond down on the allen bolts before you try to remove them, chances are you will just strip the allen head sockets of the set screws.