I know the easy answer is "we don't know how it (will) happen(s) for sure" but in the pre retcon timeline we assume Gamzee time skipped between the hatching of the cherub egg and guiding Caliborn. And we see him with Caliborn successfully in the Alpha timeline. So with Vrsika stealing the music boxes how do you think he get to the point of time with the egg?

We don’t even know if we’ve seen more than a single Caliborn Timeline, for all we know Caliborn and Gamzee are the same we follow both pre and post retcon. Vriska could’ve definitely just cloned the Time Music Boxes too, making there be two of them. A big thing about Gamzee is that his story intentionally leaves holes in the plot- It’s his thing as a Bard of Rage, he’s a wildcard, a destroyer, a jester. He takes his role as the Comic Relief to his advantage, and that’s what makes him so infuriatingly relevant. He toys with the story, whether consciously or not.