A lab turning its nose up at its food
is a contradiction in terms. Labs are not known to be finicky eaters.
You need to get the dog into a Vet for a blood work up. Various metabolic disorders can cause eating or digestive problems. Also just as likely are parasite problems. Gas-X is ok as simethicone pops the gas bubbles by lowering the surface tension on an air bubble. It's a totally inert substance biologically in the intestine. Of course popping gas bubbles doesn't quite get to the root of the problem. ;)