You need to give us more than 19 minutes to reply! lol
Anyway - it's probably dropsy. Do the scales stick out a bit - like a 'pinecone'? Dropsy is a symptom of organ failure and consequent accumulation of fluids. At this stage, the fish can rarely recover. Is it still eating?
If your fish doesn't actually have dropsy, it could be constipated - in which case fasting for a day might fix the problem. However, with a dwarf gourami like yours, dropsy is more likely. He probably has an internal bacterial infection and your other gouramies may have it too. Do you have any antibiotics to hand? It's a good idea to start the treatment now. Even if this one dies, continue the treatment for as long as it suggests on the bottle for the sake of the remaining gouramies.