I am thinking of adding a spinnaker rig (using windsurfer pole, etc) to my G-Cat 5.7, but wonder if this is a good idea since my mast does not have spreaders (diamond wire rig). Any advice???

i would think you need them with s spin. masts do very weird things (bend) with the spin up.

i have a friend who has a gcat 5.7 who flies a spin.
he has spreaders..

You can buy a spin kit for a hobie 16...they don't have spreaders. Maybe a smaller spinnaker and take it easy to feel it out...set aside some money for the possible repercussions .

You can add spreaders to your mast as well