Have you shot it yet? What were the results on target?
Rear sights mounted in a dovetail slot, like those on your Glock, are designed to be moved left or right to center the shot group on target. Many things can affect the group's location, including manufacturing tolerances inside the weapon, ammunition type/weight/velocity, grip symmetry and pressure from the shooter, and last-but-not-least, shooter skill. In my opinion, certainly shared by many others, the ONLY reason to move the sight is to get the shot group closer to the center of the target. This is usually accomplished by using a screw-threaded mechanical sight adjustment tool, or carefully tapping it left or right using a small hammer and non-marring punch.
In my experience, once your Glock is zeroed so it shoots in the center of your target with the ammo you are using, it is completely normal for the sight to be slightly off-center. Of my three Glocks, and one my son owns, none of them have the sight perfectly centered, although one is close. I have also seen one Glock, in .40 caliber, that was zeroed with one edge of the sight "hanging off" the edge of the slide, completely out of the dovetail; that this was the needed position was confirmed by several shooters who all hit center when they tried it.
Here are the choices: center the sight on the slide for pleasing visual symmetry, and miss the bullseye (or entire target) when you shoot; or, suffer the slight assault on the senses produced by an off-center sight while being able to hit what you shoot at. Choose carefully...