This is a place name acquired from once having lived at a place spelled Gill(s)thorp(e), Gilsthorp(e), Gill(s)throp(e) or Gil(s)throp(e) located in the Old Danelaw area of England.
The first element, Gil(l), could be a Scandinavian word denoting "a ravine, narrow valley with a stream" or it could be an Old Norse descriptive name, Gill(e), denoting a "servant, worker."
The second element, - thorp(e), is of Old Norse origin. In Old English, it denoted "a farm" and, possibly, a "hamlet." The Old Danish meaning was "a small village due to colonization by a larger one."
Thus the meaning could be "farm, hamlet, near the ravine" or "Gill's farm or hamlet."