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..."
When you skip a tutorial in a video game, and get stuck or lost in something the tutorial taught. A reference to Arin (Egoraptor) of the youtube channel Game Grumps, who would always skip tutorials.
"Bro, you're grumping it! You shouldn't have skipped the tutorial."
"Ah, yeah, I grumped it."