I see they offer "gas nitriding". How is the finish compared to the salt bath QPQ process?
Gas nitriding is a much lower temp process, and is usually a physical vapor deposition (PVD) process that has "line of sight" application, meaning that it won't deposit the coating on internal surfaces deep within narrow cavities or bores, as opposed to salt-bath nitriding that is submerged in a high temp cyanide salts bath, affecting all surfaces of the part exposed to the bath. Gas nitriding is also a deposited coating (albeit an extremely thin one) rather than a case hardening surface transformation like salt-bath nitriding, and it can thus be applied to a wider range of materials. Depending on the chemistry, gas nitriding can be black, and if so, it looks similar to salt-bath nitriding, but a wider range of colors and sheens are possible with gas nitriding.
Edited to add: DLC (diamond-like carbon) is an example of a type of gas nitriding/PVD. TiN (titanium nitride) is another.
