99 percent of the time when fish die, its because of us. It could have been the Iridovirus infection that infects mass bred fish such as gouramis. Many fish affected by the iridorivus infection will, in their last days, refuse food and lay on their sides, breathing very heavily. This is what you described your fish doing. Fish will also look plump or constipated. Near the very end, as the gills stop functioning, Gouramis will try to bolt for the surface, to suck in air. Unfortunately, if the infection doesn't get the fish, drowning certainly will.
I also see that you have a "plastic tank". I would switch to live as it will be healthier for the tank and the fish. Plants soak up harmful ammonia that could have killed your fish.
You also have a lot of Brush Algae. This is caused by Ammonia. Like with most algae, ammonia triggers this algae. Even very low spikes could induce it. Another big component in triggering this algae is High organics. High organics causes BA to grow, feeding on the organics. By organics, I mean leftover fish food, fish waste, or decaying plant matter.