You're right info is hard to come by - Googling produces only a few pages last I checked. They were developed for laser weapons research in the late 60's, reaching powers of around 400Kw in the early 70's, and according to Sam's Laser FAQ, eventually far more than that. I'm not sure why there is so little info available - perhaps because also in the 70's chemical lasers were developed and were much more powerful than the GDL and it simply fell out of favor.
Also, there was apparently no industrial interest in the GDL, so once the military abandoned it that was it. I suppose the gov't never got around to declassifying the info - I can't imagine why they wouldn't release more of it. The chemical lasers that replaced it are far more powerful and efficient so even if the info on how to build one fell into enemy hands they would likely be far out-gunned power-wise by current chemical lasers like the ABL and ATL (airborne laser and advanced tactical laser).
From Sam's Laser FAQ:
"Gas Dynamic CO2 Laser
(From: Mike Poulton (email@example.com).)
A gas dynamic CO2 laser essentially uses a rocket engine as the exciter. The propellants are burned to create a very high velocity, high pressure, high temperature stream of gases (nitrogen and CO2) in a lase-able ratio. As the exhaust expands through a nozzle, the temperature of the gases drop very rapidly. The lower energy levels are rapidly and selectively depopulated (aided by trace chemicals introduced into the combustion chamber exclusively for that purpose) while the upper energy levels (populated because of the high temperature of the gases in the chamber) remain energized. Presto - an instant, complete population inversion. After a time delay determined by the relaxation time of those upper energy states, the stimulated emission begins. The optical resonator cavity is positioned a certain distance beyond the throat of the nozzle (determined by exhaust gas velocity), so as to initiate the lasing process at the proper time.
These lasers are quite difficult and expensive, but produce tremendous beam power. I know this information because I have talked with an engineer who worked on the design and construction team for several of these ranging from 1 kW up to 100 MW CW for the Star Wars project. He said building a huge rocket engine and 100 million watt laser at the same time was the most fun he's ever had. "
After I graduated with my degree in lasers & electro-optics from TSTC in waco, texas in 1978. Prat & Whitney aircraft flew me and about 4 other fellow graduates out to their facility in Florida. That day I witnessed a Gas Dynamic CO2 laser blow a 10 inch square hole in plate steel that was one inch thick from a distance of about 2 football fields. The steel plate was mounted to a moving boxcar and the laser tracked it and fired and from cameras mounted in the bunker all you see is plasma vaporizing steel. At that time the system was good for 1 or 2 shots per day. That was in 1978, I wonder what they have these days
forgot to add, do some research a bit, the internet was rare as shoulder carried home video cameras. There were no digital cameras...depending how old you were in 78 you would know what i mean. Life was rough back then, we powered our lasers with dinasaurs ;D FYI Laser pointers were HeNes with cords :