What did Gamzee do to Terezi while they were on the meteor that is considered so fucking terrible?weren't they in blackroom, the "romance" supposed to be violent and unhealthy? :0

Yes they were blackrom, but the romance is not supposed to be unhealthy. And while their may be some mild violence in a kismesitude, Gamzee went too far.  He also isolated her from the rest of the meteor crew which is an abusive thing in any relationship.