The Gassy Knoll is a gas station that is often visited by truckers.
After Homer had decided to drive a truck to Atlanta, after the truck's original driver, Red Barclay, had died from beef poisoning, he and Bart needed to drive 2200 miles in 10 hours. Homer ate the most of pep pills from a bottle in a small shop and then also some sleeping pills to "balance it out". However, he eventually fell asleep at the wheel. The truck then proceeded to drive itself. It drove down a cliff and stopped itself at The Gassy Knoll.
Next morning when Homer woke up, he was so amazed that he told two truckers about it. They explained him that every truck has the Navitron Autodrive systems installed inside so they can drive themselves. The two truckers then threatened Homer to keep quiet.