Yeah. Merlin's full of anachronisms, but Gwen isn't one of them. We can't even be sure in what century the show is supposed to take place, for one thing, so how anyone would know who's supposed to have been around during the series is beyond me. You certainly can't judge the time period by the castle or the tomatoes, for example. :P

The 'Middle Ages' spans like a thousand years, but even if we wanted to place the Arthur of legend way back in the fifth or sixth centuries, there were non-white people living in Britain at that time (not only were there open trade routes that would bring people in, but there were undoubtedly Roman soldiers from north Africa who would've wound up in Britain, too).