I am guessing it's more the fact you are changing 100% of the water daily, while you have a filter system going.. or is it just aeration (air bubbler)?
If it's filtered, you should be doing less in the way of water changes. Yes, they are very bad with their bio waste and need a little more water cleaning then the average fish.. but sometimes you can do too much. In a 9.5 liter (approx 2.5 gallons) I would say do 50% every 1-2 days. The ammonia will build up quickly, but doing so many water changes can overly stress the fish, as well as there is a probability for them to go into shock if you aren't careful with acclimating them properly to both the temp and the chemistry each time you change the water.
Which could possibly explain why he is gasping so much- shock.
If it is just a bubbler, he may not be getting enough oxygen either, as well as filters for smaller tanks aren't always all that strong.
It's difficult to help, as the only true way to help him would be with a larger tank, the proper temp (comets and most goldfish should not be in tanks higher then 75*) with the proper filtration. For now, just keep doing large amounts of water changes often, making sure to dose the full amount of water conditioner such as Prime, anything that removes the chlorines and other metals.
I wish you luck.