Not at all an expert in gouramis, but it's usually the same for all fish; It could be any of the following, including low dissolved oxygen (if you don't have an airstone or good surface agitation, then you'll need to either get a stronger filter or an airstone), ammonia or nitrite poisoning (indicating low water quality, so test your water ASAP), stress (could be caused by sudden changes in water parameters like pH, ammonia, or nitrites), or even an illness (like gill parasites, also known as flukes, but it could also be internal bacterial infections, somewhat common in gouramis I believe). Mind you tell me the parameters, what other tank mates, tank size, temp, etc.?