My first kegging job was with a gun as well,
make sure when you pull a beer that you move the gun trigger to its full extent, and as fast as you can, this will avoid foaming up the beer.
Also what pressure are you pouring at?
Too high a pressure will cause more foaming, from memory with the gun i poured at something like 7 or 8 psi...
Are you pouring into a chilled glass?
Pouring into a chilled glass will also help in preventing your beer to foam.