Just installed my first gasblock a few hours ago. I measured inside the gasblock, both ends to the center of the gashole. Then marked that spot on the outside sides and top. Then measured that distance from the gas hole on the barrel in both directions to where the gas block ends and marked that. I also marked across those marks, a mark where the hole is, so I ended up with two '+' marks on the top of the barrel, showing the front and rear position of the gasblock and the hole position on the barrel, and marks on the side and top showing the location of the gas hole on the block. From there it was easy to align. Maybe there is an easier way, I don't know, I'm no expert on these things, it just seemed to be a sensable way to do it.
What I did after to check the allignment was to spray some breakfree down the tube, then look down the barrel. I saw alot of breakfree poor through the hole into the barrel so I think I got the allignment right. Next sunday I will know for sure.
Lois