Hi,
I'm not an expert, just a beginner but this is from what I learnt so far.
I don't see any visible bumps/growths outside, so I doubt it would be Dropsy, Fin/Tail/Mouth Rot, Fungus, External Parasites, Slime disease, or Ulcers.
If I had to guess then I would say it may have swim bladder problems.
Every fish has a swim bladder, it is basically a sack filled with gas, and this stops it from sinking or floating in the water. Sometimes the swim bladder gets infected, or the area around it gets infected.
The fish will then struggle to swim properly (thats what the gourami looks like).
There are many problem that can cause/affect swim bladder problems, so its probably best to take the video to your LFS or an expert, they can tell you how to help and maybe even why the fish got it. You can buy swim bladder control treatments and anti-internal bacteria treatments.
Sometimes constipation causes swim bladder problems, and adding frozen foods to the fish's diet can help.
What do you feed your fish?
However I am not an expert so I could be wrong/incorrect.
This is just what I think