So, 'gumping it' is when someone is not quite retarded, but they're like just on that line.
"I need to confess a sin, Father. I worked at a grocery store in my teenage years, I won't get specific. There was this one girl-- are you familiar with the term 'gumping it' --so while it's an ethical conundrum, it's not fucking a retarded person if you were to sleep with 'em. This will come important later in the story..."
One day, for no particular reason, you just feel like doing something. And you don't stop until you're done.
One day, for no particular reason, I decided to have a little beer. When I finished the beer, I thought i'd drink another. By the end of the night, I drank 47 beers and passed out. I sure was Forrest Gumping!