In first instance I will try to describe the current spinnaker halyard set up in my SP Mk1 No 14179 – you should be able to use this set up in most GPs.

1 The spinnaker halyard is in two parts – the first part runs up the mast, one end is tied to the head of the sail, exits the mast from the starboard side through a floating block and is tied off to a deck eye near to the bow buoyancy tank just to the starboard side of the centre line.  The length of this part will need to be adjusted so that sail is in the correct position when hoisted.

2. The second part of the halyard runs along the starboard side of the plate case, one end is tied to the floating block on the first part of the halyard where it exits from the mast.  This second part is taken to a check or swivel block fixed on the transom knee, it then runs forward to a jamb cleat about 3000mm aft of the back of the platecase, finally through a swivel block just in front of the jamb cleat.

If you want to use a pump action cleat this goes in place of the final jamb cleat.

It is also possible to have a split halyard running up the forestay, was popular up to about 15 years ago but most recent boats have been rigged with the internal 2 to 1 system described – but that is another story.

Hope that you can follow the description

Have a look at The Basic Boating Book (available in the Members’ Area of this site),  at the end of Simon Relph’s article on Fitting Out a GP14 Hull.   That article is many years old,  and to that extent is dated,  but Simon is a past World Champion of the class;  so it is excellent advice for its period.   http://gp14.org/members/members-2/all-about-your-gp14/

For more modern set-ups,  see the album of Rigging Phots,  also in the Members’ Area;     https://get.google.com/albumarchive/108162080692365982718/album/AF1QipMebfTCFU1AN_OsGxovVkoCUKLjKdzKA0AgHWS_

Oliver