I bought a GTL 21 because I got a heck of a deal on it, was mispriced, had to be, only paid 154$ new,IIRC. It was an impulse buy , I even called the wife to get "permission" to CYA. I have had little influence on her over the 20 some years we been married, but I have made her a sucker for a bargin regarding guns.
I really bought it for the laser, I had a Surefire X-200A, which I actually think is tougher, but the cost of the newer surefires..... The Glock light does not do a bad job, but not spectacular either. The laser keeps it's zero pretty good through mounting and dismounting(on the same gun of course) but in bright sunlight it is useless, as is about any laser, though I have not tried a green one, thought I was going to when one of my students had one, but he forgot it on our range day. I did have a bulb fail pretty quickly, had to be a defect, because we have since) used the light to extend sessions into the dark when one of my best friends comes to town(he has no place to shoot, and I have built a nice range on a buddies private property, so the 2nd bulb has been through several sets of batteries. The light comes with one spare bulb, and the first order of business should be to get several spares, I had a time finding one(but did not think to call Glock direct, duh).
I have used an aluminum Streamlight on a friends duty gun, but we did not shoot that long after twilight, but it seemed well built, and he had been working for about 8 months and his department uses light dedicated duty holsters- no problems.
The Glock light is a little more streamlined, the bulk going down, rather than to the sides, I prefer the Surefires momentary on-push forward on the switch, the Glock, you push down, which is the same motion to turn it all the way on. The GTL 21 has a selector on the bottom to choose;off;light;light/lasser and laser only. The manual warns to leave this switch in the off position when not in use, it must draw some current, I should test it and see if does run the batteries down. I think the surefire is the benchmark, the streamlight not too far behind, and Glock a bit behind that. A major draw back is the LEDs are so much better now days, and not having to worry about bulbs is great. If I recall, the Glock light only is pretty inexpensive, and would be way better than some of the chinese junk coming in, but, a chinese light with an LED....but I would still worry. FWIW the Surefire stays on the bed gun- the Glock gets taken out and shot, mainly because I am lazy and dont want to climb steps and have to remember to take the surefire back up, and the laser is a good training aid for students. I would not have bought it had it not been for the mistake in price(and I did ask if it was used because of the low price, so I kind of gave them a heads up...) Like someone said above, this is not a timely reply, but I just ran across the thread.