What else is in the 100 gallon? Is it cycled/established? Are the water parameters any different from those in the 20 gallon?
Before I actualy try to answer your question, you need to be aware that 20 gallons is not adequate for any number of kissing gouramies and that they grow to a good 10" if cared for properly. You are going to have to move the other kisser out of the 20 as soon as possible as well (but lets see how things go with the distressed one first).
Back to the point - considering the stress the fish must have already been under due to the bullying, my guess would be that the mvoe has caused it the distress you are describing. It may be a difference in pH, temperature or perhaps particularly high/low levels of ammonia, ntirIte or nitrAte. Any sudden changes to the environment could have shocked the fish. Besides checking the water to see if the problem can be fixed, there isn't much you can do about shock like this.
If, however, the fish is showing symptoms of disease (eg: bloated appearance, stringy, white poo or pineconing of scales), then you can certainly treat for it - but you'll have to provide a much mroe detailed description if that's the course of action you think you need to take.
Personaly, I think it's probably just shock from the tank change (check the pH especialy) and the fish may still recover if you are careful not to stress it further. Next time you mvoe a fish to a new tank, first acclimatise it slowly by adding very small amounts of water to a bag/bucket containing the fish' original water. Especialy with gouramies, fluctuations in pH can be deadly.
Hold on - I re-read your post and you're saying that the other fish was beating this kisser up 'because he was sick'? I should first point out that the reason the fish is sick is probably that the other was bullying it (stress leads to disease) - not the other way round

Kissing gouramies are territorial and, in as small a tank as that, it wouldn't be surprising for one fish to dominate and bully the other.
Regardless, if the fish was already sick before you moved it to the 100 gallon, the problem can't be shock. Judging by its current behaviour (and the sort of diseases gouramies are prone to), it probably has an internal bacterial infection. however, it would help if you gave me some mroe information - for example, how's its appetite? Is it bloated; do its scales seem to stick out? Any sign of red streaks on the fins or body? Does it go up for air at all? How is it color-wise? Are its fins intact? Any red, white or black edges to the fins? Do the eyes look normal - no buldging or reddening? Before it started lying on teh bottom, did it swim in a head-down position? How was its appetite then? What color is its poo? What do you usualy feed it? Have you treated the tank for anything recently? What other fish are in the tank? Have any been sick in the past? Anything you can say will help. Also, if you can get a picture, that may be useful.
What I would do right now is move him into an isolation tank in case the problem is the 100 gallon. If you can test your water and the difference is not in water parameters, I would still reccomend you do a water change (a small one) just in case something else has gotten into your water (it would, however, have affected any other fish in the tank as well if that's the case).
However, if he was already sick, the problem is probably an infection which, by now, sounds to have taken hold quite strongly. Again, isolating the fish and treating it with a broad-spectrum antibiotic may help but it's probably too late. You can also treat within the tank if you can't isolate.