Products like Gas-X are just fine to use in dogs.
If this were my dog, I would dose in proportion to the dog's size based on the dose for adult or child's weight on the packaging. (ie: if the dog weighs 50 pounds, then base it on the child's 50 pound dose).
If the gas in your dog continues, or if it is accompanied by vomiting, diarrhea, painful, bloated stomach or other 'unusual' symptoms, then you should have your dog seen by a vet right away as this can be a sign of a serious problem called bloat.
Hope this helps!