Using a drill bit upwards in the bottom of the gas block, enlarge the gas tube hole to fit/align with the gas block passage (note: only enlarge the bottom tube opening, and not drill/make a new hole into the top of the tube).

On the gas block to barrel, mic and mark the center of the barrel gas port to the barrel shoulder.  Now mic the gas block to center of its gas passage. On some Gas blocks, the gas passage is set for installation of the front hand guard clip.  If in this case, you are not using a front clip, then you would set the gas block slightly off the barrel shoulder the needed space of the hand guard cap, and then using the barrel shoulder mark you made, align the gas tube to the shoulder mark when setting the gas block.

Now the last item to bring up, and really the first to check, is confirm that the barrel gas port is top dead center of the upper receiver.  If the barrel was not installed correctly indexed, then when you go to index the front sight at zero, you will miss index the gas block passage against the barrel gas port, blocking the works.