One of Forrest Gump-like disposition, lacking in basic cognitive and linguistic skills. Possessing a knack for having a constant look of bemusement on their face.
Also used to describe the language spoken/typed by a person of Gumpish disposition.
Matthew: "he ring up late not and got tell there is no problem with it."
Smithie: "Dude...that sounds so gumpish, what the hell are you talking about?"