Sorry about the crash. I tried to replicate but it didn't happen when I did it. May have been a freak occurrence. Anyway, you don't need to go back to Gongaga after delivering the letter, he won't be there anymore.
Yeah, I couldn't replicate either. So it probably was one of those random occurences.
But I did just run into a new bug.
After finishing Rocket Town's Huge Materia mission, when on the Highwind during the break in the next cutscene, if you try to leave the screen instead of talking to Red XIII, when he talks to trigger the next scene, Cloud tries to run into position and gets stuck on Cait Sith or the Highwind Pilot (depending on where you tried to exit) and it leads to a soft lock. It's a glitch most people won't run into since they'll talk to Red instead of leaving, but I wanted to see if you'd be allowed to leave lol.