You could try a plastic razor blade (ScrapeRite Polycarbonate Plastic Razor Blades
), but suspect you may need to go the chemical route. Oven Cleaner definitely works and doesn't hurt the gel coat, but I'll admit it made me nervous the one time I did it. Use gloves -- -- it's very caustic. I might also suggest a rag with a little acetone to clean things up, but don't go crazy -- acetone can soften gelcoat (also use good gloves -- acetone is nasty stuff). Make sure you're doing all of this on a well ventilated area and be careful of any wind-borne overspray of the oven cleaner. It works quickly, btw (not instantly, but very quickly).
+1 for masking off the area. Suggest a long piece of drop cloth material taped along the bottom to direct run-off away from the transom and onto the tarp which you placed underneath
If you use a plastic drop cloth, it may not react well to oven cleaner -- test it first (it may just melt, creating a nasty mess).
BTW, your boat is on the hard, correct? Oven cleaner in the water makes fish taste bad and is bad for your karma... and your brain.