If his back end is floating, he probably needs a good poo. As for the gills, if they look like they're getting fungus, white fluffy bits, he should be given salt baths. If they just look like they're shrinking, this could be due to lack of need. The more oxygen in the water, the less gill space they need. A close up picture of the gills would be very helpful.
Have you tested the water chemistry? You need to test for ammonia, nitrites, and nitrates. PH, GH, and KH are less important, but should also be kept at moderate levels. Ammonia can burn axolotl gills, and if you have not cycled your tank, and are not doing frequent water changes, you could be poisoning them with their own waste.
I suggest taking a sample of your water to the pet store and getting it tested. Most will do this for free. Then get a dry-tab test kit, not the strips or the liquids, and use this until your tank has cycled.
Have you read
www.axolotl.org yet? This will give you a lot of good information for starting out.