Assuming Gwen has shamelessly flirted with him offscreen, and I do, I feel like she'd be oblivious he's a vampire so it's something that'll come as a surprise.

As for the Big Revelation, I'd love for Gwen to be involved somehow but I don't really know how, since she's only really known one vampire and he never went all hermit/crazy. Even if she came across a bunch of Extras I don't think she'd get all detective about it, and she'd leave that to other people.

I DON'T KNOW IS THE ENLIGHTENING CONCLUSION.