I doubt you'll find any full-length DVD's on Gnutella. There are a few on eMule (check here) and lots in the newsgroups:
alt.binaries.dvd and alt.binaries.dvdr
You'll need some sort of software like GrabIt, BNR2, Forte Agent, NewsLeecher, or NewsBin Pro. I personally use BNR2, but if you're just getting started, you may want to try GrabIt or one of the others first. You'll also probably need to subscribe to a premium news server, because most ISP ones won't do for large binaries, like DVDs.
The movies on Gnutella are almost entirely DVD Rips converted to DiVx format, which I doubt you'd ever be able to convert back to an actual DVD. Even if you did, it wouldn't look or sound nearly as good as it originally did.