By "pinning" do you just mean inserting the roll pin into the gas block?
If so, it is not difficult but requires either assistance in stabilizing the block while tapping it through or a good ar block on which to place it. Good videos on YouTube for help on the process.
Ahhh... Was not thinking in terms of barrel milling for sight tower block with pins... I don't have the machinery to do that... Would have to buy a pre-installed barrel/tower if I were to build one of those... Sorry to have commented based on pinning a gas tube into the block