That's totally fake BS. When I was 15 my best friend accidentally swallowed gas while trying (he'd never done it before) to use a piece of garden hose to siphon gas out his older brothers car. It was about an hour before he got to the hospital and the doctor estimated he'd swallowed about 1/2 cup. They filled his stomach with charcoal and did everything medically possible to get out of his system.
During the next 2 years he underwent 6 surgeries to remove damaged parts of his esophagus, stomach, spleen, and intestines. He died at the age of 23 from the effects of drinking a very small amount of gasoline.