Stinky gas can be caused by anarobic bacteria digesting the protein in the food, so a good way to handle it is to add a good multistrain probiotic to the food, to rebalance the probiotics in the gut, and a digestive enzyme to help the dog get everything out of the food until the right balance of probiotics can pick up some of the slack.
Switching from Purina to BB Wilderness might have been too big a change in nutrient levels for your dog to handle. Maybe try something like NutriSource. It’s pretty well tolerated and easy to switch to.
