One who gets the gist of a subject matter and knows enough to be dangerous with that subject in conversation or writing. One with proven gisting abilities. A gister can participate in conversations with subject experts without getting lost along the way.
Joe is such a gister — those research guys think he really has expertiseness in selective serotonin reuptake inhibition.
If it's complicated and you don't have a lot of time, ask a gister to write it.