Hi and welcome to the Hub!
While alot of owners say their Boxers have gas it isn't "normal" it's a sign that their system is out of whack. It is common for Boxers to have sensitive stomachs and sensitive stomach can = gas
I would keep trying different foods, making sure to do a slow change over when trying a new food. I find it is not really possible to recommend a specific food because each dog seems to do well on different foods. I would suggest sticking with a good quality food and trying different protein sources and maybe even a restricted ingredient food. Digestive enzymes are also beneficial. It may take a while to find what works but with a little patience you'll get there.