I've sailed the Gemini quite a bit in the Pamlico Sound.
I used a 40 cm Lessacher Design Formula Duo fin that really worked well with big sails
(8.5m2 Retro on the back, 7.5m2 Retro on the front) and small sails (4.8 Huckers front and back).
I also used a Tangent Dynamics Reaper 42 cm with good success.
Any good 40 cm or larger weed fin should work.
Hope this helps,