If your Glock is shooting to the left, (and you are right handed) it is very possible that you are squeezing the three remaining grip fingers on your right hand as you pull the trigger. This will cause the pistol to move ever so slightly to the left.
To test for this, hold up two fingers on your left hand upright as if simulating a pistol grip. Grasp those fingers with your right hand just like you would grip your pistol. Now, simulate pulling the trigger with your right trigger finger. The vast majority of the world will notice the tendons in their right palm tightening while you simulate pulling the trigger. This causes you to slightly roll the pistol to the left while shooting.
This is NORMAL as our hands are designed to grasp things using all fingers together. So, pulling a trigger, and expecting none of the other fingers on that hand to tighten along with the trigger finger isn't the way we are made. The solution it to practice exercising your trigger finger with as little movement in the other fingers as possible. Over time, you will get much better at separating out the tendency of working together.
It is sort of like trying to do that Mr. Spock finger split thing without practicing it first...our hands just aren't designed to do that, they have to be trained.