Man, that's a tough one because 'Danganronpa' fandom has moved around a lot. Most of the big GontaGoshi stuff I've seen is on AO3, obviously. The tagging system over there makes it super easy to filter for that specific ship, and you can find everything from fluffy first-kiss stuff to wilder AUs. The quality's generally pretty high, too.
I used to find a bunch on Tumblr as well, but it's more of a scattergun approach—you're digging through reblog chains and hoping the links aren't dead. Some writers cross-post, but not all. Wattpad has some, but in my experience, the stories there skew a lot younger and the tags are a mess, so you have to wade through more unrelated content to find the good ones.
A real deep-cut spot? I stumbled on a surprisingly active thread for them on a niche 'Danganronpa' forum once, but I can't even remember the URL now. It was all text-dumps in the replies, very old-school. Honestly, AO3 is your most reliable bet for volume and consistency these days.