^They're aright.
All of those are possibilities actually. As far as I've understood from the Gigantour forums, Mike said in an interview that Opeth is definitely on Gigantour, as did Michael Amott from Arch Enemy. Anthrax has said they'd like to. (Though didn't say for sure, like Arch Enemy and Opeth.) Overkill left a comment on someone's myspace saying "Gigantour is a go." (It was quickly deleted afterward.) I don't know exactly about the rumors surrounding Sevendust, but I think they're a strong possibility too. Gary Holt of Exodus mentioned that from September to October, they're busy doing U.S. stuff that they're sworn to secrecy about (Shawn Drover subsequently said Exodus isn't on it, but I don't believe it, otherwise Dave would have said it. Probably just want to still try to surprise people.) 3 Inches of Blood I could see being on it, except that they've done a whole bunch of U.S. tours thus far, and I haven't heard anything about them being a possibility. Same with In Flames - I think they're doing Sounds of the Underground now, even though it's over before September, would they do Gigantour right afterward?
Altogether, it's possible that thing is fake, just as possible that it's a leak. Hopefully a leak, because I like all the bands on that except Sevendust, and would be ecstatic to see everyone besides Megadeth (last year's Gigantour) and Overkill (don't particularly love).