So... I'm the "hinge" in a V, but I would *absolutely* tell either of my partners if (when) I have had trouble. It only seems fair to say "hey, this happened, I had a blah week" or "I need an evening alone to just deal with the turmoil that x caused".
And seriously, not telling K is doing her a disservice. She should be able to make her own choices in this situation.
As for whether you ask G to tell her first? I would tell her that you're going to have that conversation with K, and if she wants to tell her her side first, that's fine, but that it's going to happen either way.
Best of luck!