How would pre-retcon gamzee get to the cherub universe at all, and how was there ever a pre retcon caliborn.

Pre-retcon Gamzee was killed by Kanaya, he never reached the cherubs. The Gamzee that raised the cherubs was post-retcon. There is no pre-retcon Caliborn since he couldn’t have existed without the recon because the game over timeline didn’t result in a new universe.