Okay, try this (I had the same problem):
Low left means you may be slapping the trigger. The trigger of the Glock takes some getting used to. I had this problem and still do to some extent, but not anywhere near as bad.
Because of the trigger, your support hand (left hand if you're right handed), is not holding the pistol enough to compensate for how much your middle and ring finger on your dominant are pulling down on your Glock. Mixed in with the angle of the grip, and you'll end up shooting low/left.
I used the Ayoob Wedge, and it changed everything. Do a Google search for it.
In addition, I shot AR-15's and 1911's a lot throughout my life, and ALWAYS put the meaty portion of my trigger finger before the first knuckle on the trigger. Don't do that with a Glock. Use the actual joint indention itself.
So, try these two things and report back:
1. Ayoob Wedge
2. Use first joint, not meaty portion of index finger.
Find a shooter that teaches... and take a class. The front sight will tell you everything you need to know while dry firing. Penny not needed.
I know you've already gotten a lot of suggestions..... My recommendation differs from many posters here. If you are shooting right handed and your shots are going to the left I think you don't have ENOUGH finger on the trigger. In other words, you are pushing the muzzle to the left. Are you shooting using your fingertip? If so, try shooting from the first joint on your trigger finger. Also, what helps me is if I increase my grip with my weak hand....this also will provide more support and counteract the tendency of the muzzle to go left.
Also, try some dryfiring at a mirror....say in a bathroom. Watch the muzzle very closely and see what it does when you press the trigger. You will probably see a small movement of the muzzle to the left. Try moving your finger further onto the trigger and see what happens. You want the muzzle to stay stationary when you fire....using a mirror can help you find out what you're doing wrong.
You might be jerking the trigger:
I solved that problem with lots of dry fire with a laser. I could see the way i was pulling the trigger. It took changing where on my finger pad I was engaging the trigger.
Larry Vickers talked about this on an episode of tac tv recently.
There are drills (DF) to help isolate the trigger finger and will expose what might be going on during live fire that may not be noticable during slow deliberate dry fire.
If you can't do this deliberately without disturbing the sight, there is no use in going on to the drill.
Thanks everyone! I am hoping to get to the range tomorrow and will report back.