All the Konis that I know of are just normal hydraulic. The only reason that other manufacturer's 'gas charge' their shocks is to prevent oil 'frothing', Koni's superior design doesn't require it. (No true damper should provide a spring component also)
The 'gas charge' is actually a gas compartment at the bottom of the shock, separated from the hydraulic oil filled section by a floating piston. The idea behind the that is to keep the oil pressurised and therefore less likely to froth (liken it to a softdrink bottle- shake it up, but keep the lid on and all is well. Release the pressure, and bubbles all over the place)
I haven't come across a Koni that isn't adjustable in the rebound. You usually need to remove the bump-stop, compress the shock fully (a nut at the botton of the shaft locks into a recess in the casing) rotate clockwise for tighter rebound, ccw for looser rebound.
For road/race use they're pretty good value for money. (I've had a set on my '92 Falcon for at least 6 years now (150000km+) and they are still brilliant)