First, narrow your question. What kind of Gas-delayed blowback? That used on the HK P7 or something like that used on the Steyr GB? (If you go researching around firearms history and patents, you may find others). Both the P7 and GB have antecedents in early 20th century designs.
I seem to recall reading something saying the gas-delay concept goes back to the late 19th- early 20th centuries, around the time Browning, Luger and others were developing new semi-auto pistol designs around metallic cartridges.
I also seem to recall some literature from H&K discussing the gas-delay concept used in the P7 and mentioning it as a refinement of a much earlier patented system. Maybe try over at the HKPro website and see if it has any information.
Try a patent search on the U.S. Patent Office website. Might take a bit of creative searching, but should be able to come up with something. I'd look first for HK's patent covering the P7 and look at what earlier patents it references. Also, look at Steyr's patent on the GB and what patents it references.
