My recommendation doesn't hit all of your points, specifically the Gaff rig element.
I had a
trailer sailor for some time and my biggest obstacle to getting out sailing- by far, wasn't launching it, or trailering it or
weather. It was stepping the
mast, especially if you will be alone.
Based on that, I would suggest something fairly small with a lightweight
aluminum rig. The smallest
Hunter, the 15' seems to meet most of your criteria, I'm not sure any Gaff rigger would be very easy to rig for an afternoon sail.
A 15 should be big enough to keep you dry, carry a couple of freinds, even carry some camping
equipment and an
outboard if you desire.
Just writing this response is making me want to load a
dinghy up with camping
gear and go sailing in the desert.
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