Any weapon left to you in a will by a relative. Not necessarily a pistol, or even a service weapon, but usually. Generally left with an instruction not to use it for anything retarded. Usually in very good condition.
So named because of the stereotype of a grandfatherleaving his favorite grandson his service pistol.
abbr: GSP
Guy1: Hey, where are you going.
Guy2: Taking Grandpa's Service Pistol to the range...
Guy2: But... that's a shotgun.
Guy1: Yeah, "Grandpa's Servoce Pistol" is just what I call any weaon left to me by a dead family member.