Ok, I did a test with some paint stripper from Interlux that specifically said it was safe for gelcoat/fiberglass. This stuff seems to do the trick on the non-skid. After about 10 min I was able to scrape the surface down with a putty knife.

Will do the entire deck once I've got a free weekend. It'll need to be re-painted where the non-skid was...I'm thinking of masking off the same areas, just a little wider at the edges, and hitting it with a contrasting color (a deep blue or dark grey for example). Apparently Interlux makes a non-abrasive non-skid additive for top decks...anyone know something about about it?

Mike.