I thought about mentioning the GLFS before, but the OP was asking about a 19" barrel which does not by itself require one. However you bring up a good point and that is that the gas locks that incorporate a dovetail for the front sight vary in height from one manufacturer to another, and thus require a different height front sight post. This is why they all make their own front sight to go with their locks.
However, the required front sight height is a function of Trig. If you shorten the sight radius but leave a standard height front sight, you may not be able to lower the rear sight enough to compensate, SO you have to use a taller front sight.
Think about it this way... Barrels are always elevated X degrees to hit the target. So draw a line on a peice of paper elevated at a slight angle; this is your barrel. Now draw a level line from the muzzle back to the shooters eye. Now draw a front sight that intersects the horizontal line to the shooters eye. That's your standard front sight. Now let's cut 6 or 8 inches off of your barrel and draw another front sight that intersects the line. It's taller right?