I don't think Peto Bismol helps with gas, it can help with other digestive upsets, but not gas.
Taken from this site:
Controlling Intestinal Gas in Dogs
Although gas production is a natural part of the digestive process, some pets produce more than others and are not always fun to be around. Here are a few ways to reduce flatulence in dogs:
1. Slow down dinners. Flatulence is often caused by air that is gulped down when pets eat too quickly. Putting a large object in his food dish will force your pet to slow down when eating. The object should be something that is too large for the pet to pick up in his mouth. Try something like a ping-pong ball for cats, a baseball for toy dogs, a softball for medium breeds, and an even larger ball for large and giant breeds. If you have more than one dog, feed them separately to reduce competition for food. You may also try scattering dry food around the house and/or yard so the dog will need to 'forage' for it.
2. Buy an elevated feeder. Dogs that do not have to bend over so far to eat swallow less air.
3. Take walks after dinner. Light exercise aids digestion and works out the gas while you are outside. If your dog eliminates during the walk, even better.
4. Switch food. Your brand of dog food may be the culprit, particularly if it is high in soy. Switch over gradually so your dog's digestive system can adapt to the new food. Substitute about 25% new food for the old food each week so that by the fourth week your pet is on 100% new food.
5. Add Acidophilus or Prozyme to food. These products help in the digestive process and may help eliminate gas completely.
6. Watch what they eat. Too much food at one time can cause gas, as can eating out of the garbage, or too many table scraps.