I believe Edmund Storms has proposed a theoretical model that can explain the lack of gamma rays. Dr. Storms worked closely with Michael McKubre on LENR. A similar pairing, of Peter Hagelstein and Mitchell Swartz, has also come up with some theory that explains the lack of gamma rays.
There have been a few reports of gammas.
https://www.mail-archive.com/search?l=vortex-l%40eskimo.com&q=gamma&start=100