While researching this I came across this bit of info on the GP.
GP is is for Gaffer or General purpose shotgun. According to Greeners book the exact number of GP shotguns made is impossible to be determined as the serial numbers of these shotguns were not recorded until 1968.
If your gun is a Police gun then the serial number look up I did for you is correct. If your gun is a GP then it's not possible to determine the date made by serial number.
Two models of GP are referenced. Standard model has no checkering and a barrel band. The second model had a better finish, checkering and looked more like a sporting weapon,
The Greener Story by Graham Greener has a lot more on this gun and other Greener arms. The book is a bit spendy but I have found it quite useful in my Martini hobby. There are contradictions in the book. The text makes no mention of a MK designation in GP's. It refers to the checkered version as Model number 2. You should be able to look at the side of the action and see what it says to determine if you have GP or a police gun.