Loyall brand puppy food giving my puppy gas!?

I have a 9 week old great dane, who eats a TON! I was feeding him Purina, but the vet pretty much slapped me on the hand and asked me if I was trying to kill my dog what that junk. SO. I walked around Atwoods some and found a brand that had a ton of samples. So I grabbed some puppy food samples (the brand is Loyall, two L's at the end) and he seemed to like it. I ended up buying a big 40 pound bag for him. But now he's got awful gas! He pes gas almost every five minutes or so, and he can clear a room. He didn't do this on purina, should I find another brand or is this just his digestive system developing?