When Kirk encountered them they appeared to be completely unknown to The Federation. Not even a rumour.
Incorrect. I just popped over to Paramount+ and watched a chunk of "The Arena". Early in the episode, when Kirk asks Spock about what creatures they might be facing, Spock notes there is no official information. "Nothing scientific," he says. But he does refer to "space legends" - which I think qualify as rumors, no?
Also, SNW is not the first to change Gorn continuity slightly. Captain Lorca had a Gorn skeleton on board Discovery.