I did exactly what you are suggesting and got an ajp rail for the GP to fit my Mk4 target.
Sadly they do not fit. Have a look at your block , where the magazine fits in. See how it bevels in. This prevents the rail from either sliding forwards or backwards which means it will not fit.
Jonno was very good and let me have a full refund.
He did say that some people have had their blocks machined to get rid of the fouling bit, but i wasnt going to do this on a 4 mnth old gun.
I ended up with a Barry Taylor rail, although i now use a dovetail to picatinny adaptor and warne qrw mounts so i can swap my scopes easily.
I would eventually like to get something like ajp rail, cut it in half long ways so the bottom is in two bits and have a top picatinny rail that would the screw down onto the lower sections.