There is no reason why you shouldn't fire decals in a gas kiln. The main thing is to have a very slow temperature rise to 400-450.C so that all the combustibles can burn out. At all times keep the kiln in an oxidizing atmosphere to 800.c. You need to keep a clean atmosphere so even over-oxidiziing is advisable. For many years I fired both lustres and decals in my gas kiln very successfully.