J.G.
Since you are asking you obviously already know that tummy time is important. I am a pediatric PT and yes being gassy or having reflux can definately make it uncomfortable. We see alot of kids who dislike it this day. There is no proven study but we think it's bc kids aren't sleeping on their tummy's so they aren't used to the feeling. Anyway...just keep at it. Small bursts as many times a day as you can. Tummy time on you counts and the ball idea the pp gave is also a good one. Here is a great handout that can give you some more ideas/info. Just click on the tummy time link in the first sentence and it will give you a pdf handout that you can print out!
http://www.choa.org/default.aspx?id=4892