As much as my inner cheapskate would hate it, I would probably scrap the tire. If you keep it, put a boot on it and only use in the rear. I have booted and continued to ride tires with similar cuts but I never felt secure, waiting for a blowout to occur. A compounding factor, with this particular tire, is that I also don't like Conti Gatorskins. They have worn well for me, but I don't like the way they ride.

That said, I don't blame the tire for the cut. Stuff happens. I have gotten similar cuts on wide range of tires, sometimes with only a few miles and sometimes after a couple thousand. The one that hurt the most was a Rivendell Jack Brown in which a large rock cut the sidewall right at the bead, and it was unrepairable. I was able to limp to a bike shop using a boot, but just barely. Another time, a piece of glass cut a nearly new Conti GP 4 Seasons on the tread, and I booted it and continued to ride for 2,000+ miles without another flat.