A gathering? A party? It would be pretty difficult to organize something like that for a typical ctb, at least the way society is now, though. It's a really nice thought though, maybe in the future we'll have less of a stigma around self-decided death and people will be more understanding, and a person who is chronically suicidal will have family and friends understanding enough to know their plans and acknowledge that they're not changing, and to have one last celebration of the departed's life. This would be awesome. Society is very far away from reaching this point, however.
The best we have now are suicide partners. I plan on catching the bus with @TransTaxEvader this year. I doubt organizing a suicide party is something that will be possible in our lifetimes.