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."
Playing an incredibly popular video game, often for hundreds of hours, and learning nothing whatsoever.
This can either dramatically lower your enjoyment, or raise it far higher than it should be.
"I just played Blank for the past week! It was so much fun!"
"Nice! What did you think of Story?"
"What? Oh I don't give a shit. I was just Game-Grumping."
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..."