The G26 and 27 haven't any rail for front outside mounting. I installed the the Lasermax guide rod sight on my G27 a couple of years ago. It installs internally by replacing the original guide rod in a matter of a few minutes. My gun has night sights, but in the dark you'll never get to line them up to shoot. It turns on and off easily and testing it in low light, in my case, hit within 4, or 5 inches at 25 yards. You have to learn to keep your hand steady though, which will take practice. Also, since it is internal there is no need for a special holster. Some say they're useless, but it's there if I ever need it. The drawback is it's price. This time of year sales are run and I got mine from OpticsPlanet at the cheapest price.
Locusts is right well pretty much any way. I'd opt for the night sights, that's what I have on my Glock 27. But I think and I stress think crimson trace makes a laser that attaches to the trigger guard. I wouldn't touch something like that with a ten foot pole though. To me anything other than an ir laser is utterly useless. Learn to point shoot and buy a flash light you'll be surprised how accurate you are at self defense distances with out any sighting aid. Lasers are Hollywood trash, unless it's something like a dbal2 combined with night vision.