gc = g潮 [g cháo] = 高潮 [gāo cháo] n.
Climax. The letter g is used to replace 高 [gāo]. Although 高潮 [gāo cháo] also means “orgasm,” this Internet slang term is not necessarily used in a sexual context. Often it simply means “the funniest part” or “the most unexpected part.”
When passing by a flower and bird market [an outdoor market that sells plants and pets], I heard a parrot talking. I thought it was fun so I stopped to listen. Cut…It said, “Welcome!” I didn’t think it was a big deal, (but rather) normal. Then, the climax came. It stunningly shouted out, “Yo! How did I learn to speak!?”