Yeah, what TL asked... It almost looks like it's a wound & scrape from jumping & hitting the top. If so, the best thing you can do is change water, lots & often as you can. Keeping the water very clean can help with healing...except for the iridovirus (?). Nothing helps with that, sorry.
The pic doesn't look like the "gourami disease" to me. Those fish often look bloated. Sick fish often hide. I see you're not a total noob...
Not seeing your dwarf? gourami makes it harder to guess. Does your fish have a red spot +/or streak like the pic? Antibiotics might be appropriate...but we need to see your fish & know more about your tank. Lots of questions!
What size tank? How long has it been set up? Did you cycle it before getting fish? If so, how? What other fish do you have & for how long have you had the gourami, etc.? How often do you change water & how much? A tank pic, or at least a detailed description can help too.