The word derives from "give me," as in, "Will you give me that one?" Many golfers play using gimmies for any short putt. The unofficial rule is "inside the leather" - that is, if a ball is closer to the hole than the distance from your putter head to putter grip when laid flat on the green, it's a gimmie and you can pick up.
The key word there is "unofficial." Because gimmies are never allowed under the official rules. In match play, an opponent can concede a putt of his or her own volition (without being asked), but the rules never allow gimmies.

