It's actually a surprisingly broken tool. Had a game yesterday, one city, going for culture win (accidentally won diplo instead). Egypt to my east (local superpower), 3 other civs to the north and east.
Egypt was giving me the evil eye, so i denounced them. I usually don't do that, favoring trading partners, but i only had 3 resources to sell and they weren't going to give a good price so i figured, why not.
Turns out this gave me several gamelong allies. Egypt quickly decided to get rid of me, everyone else denounced egypt, my buddies on the continent went to war against them, and i was left alone to culture it up by everyone except egypt. Usually i end up friendless by the late game, but not this time.
Also, the GG building is awesome under the right conditions. Egypt had to march through mountains to get to me, and there was only a one tile opening. Plop a fort down and nothing got past it, not even when they sent a gunship against my rifleman. Probably could have sacked half their cities if i had bothered to make some artillery instead of buildings, but eh. Next time.
A thought occurs. Could it be that my military inferiority (i had 4 land units at most at any given time) could be part of why the other civs sided with me? helping the innocent underdog against the evil warmongering Egyptian overlords? Or did i just luck out?