It probably is air coming out. The tank is supposed to be vented, so that as you put in gas, the air comes out. If it doesn't vent the right way, it will force the gas out.
If it does this while you are putting the gas, then slow down on the pump a little bit. My old Jeeps will not allow me to put in a lot of gas at a time, I have to manually run the pump and keep it slow. It takes time, but if I try to go faster, it will splash back at me. It is vented, but the fill tube is just to narrow to handle the volume of gas going in, so it fills the tube and comes back at me.