Is the gourami the only fish affected? Is he rubbing or scratching on decorations in the tank? If so, he may have parasites. Can you see anything that looks like a parasite? If not, it may be a bacterial infection.
You could make a
QT tank with a 2-5 gallon plastic tub (new, clean, never washed with soap), an airstone, and a 25 watt heater. Put the gourami in here with some plastic plants or
PVC elbows for hiding places. Add the meds to this tank, and watch the main tank for any abnormal signs on the other fish.
I don't know if it will go away on its own. You could try doing 2-3 water changes first and see if that makes a difference. Sometimes very clean water will help a fish heal on his own. It's hard to tell if it will spread without knowing for sure what it is. If you do see parasites, for example, you probably would have to treat the whole tank. If the other fish look well after a few days and the gourami isn't getting better, then
QT the gourami and try an antibiotic.
Some good antibiotics: Jungle Fungus Eliminator, Maracyn 1 or 2, TriSulfa, Triple Sulfa, Kanaplex, Ampicillex (may be hard to find this one). Some parasite meds: Jungle Parasite Clear or Parasite Guard, Clout, Metronidazole, Fluke Tabs.