The Town side had a strong Scunthorpe connection, with former United goalkeeper Mario Ziccardi beaten first by a Dave Cameron header.
Grantham's equaliser thirteen minuted from the end came through former Iron striker Gary Bull, a player who was profilic for United's reserves but only managed one senior goal for the club.
When the final goal came five minutes later it was another Iron old boy who did the damage, Jimmy Neil this time the scorer.
Neil spent two years at GP after joining from Grimsby Town in 1997, he featured a total of seven times in that period.