Scum killed Gorlak during the night, so in a vacuum there would be no way for GoG to verify her claim, so that's how the situation should have played out.
So yeah, it's cheating. I don't think it was done maliciously, just something to be clarified for the future.
(Technically you could still fake claim something like that. For example GoG and Gorlak speak outside the game in a totally unrelated conversation, and then GoG uses information from that conversation to back up a fake gossip claim in game. Whatever the rules currently say, I think we generally agree not to play at that kind of level...)
((...speaking of which, I did briefly consider running a game that isn't limited to its thread, or that doesn't have a "main thread" at all...
I don't think people would like it.))
No
You're using evidence to prove something took place that's well outside the boundaries of the game.
Beyond being outside the game it is direct confirmation that your conversation took place which isn't part of any of your roles.
[Rule 5]
That rule is actually about stalking/tracking people's activity, although I guess the way it's worded you could use it.