Probably Fungus. I would do freguent water chagnes and go pic up a good fungal medicine from your lfs. Dont do frequent water cahgnes after poutting the med in. Follow directions on the box of bottle.
What type of Gouramies are they? Do you know what sex they are?
It does sound as though your Guppy has a fungal infection. Interpet No. 8 is good for fungal infections, so worth giving it a try. Have you got somewhere that you can quarantine him just while you medicate?
As Kribensis said, poor water quality can lead to infections. How long has the tank been set up?
You may end up with problems having all male Gouramies, I'm afraid. You are best having at least two females to one male and the less males kept together, the better, as they can be very aggressive as they mature.
You are able to keep the smaller Gouramies in that tank but you would be best having one male and two females to prevent any problems in the future.
Good luck with the treatment.
No problem. You aren't the first to get bad info, we suffered exactly the same when we first set up a Gourami tank.
Good luck and let us know how you get on.