An implied result. Something expected, owed or earned. Expecting a gift. Short and slangy for ‘give me’=gimme. Often used in golf when a ball is very near the hole so there is no need to hit it in as it is so easy, the golfer asks for a ‘gimme’ instead of putting.
“My golf ball is very close to the hole, so that is a gimme, I don’t even have to try to knock it in the hole”