It's a cluster fuck how they handled the Anniversary multiplayer. Sometimes when I try and play Reach it launches the Anniversary MP -- it's really quite annoying. Don't know why it couldn't just be a DLC code that doesn't require the disk.You're talking about a company that released Halo 1: Anniversary, and ran out of space to include the Halo 1 gametype on the disc. Meaning you cannot play Halo 1 Slayer on Halo: Anniversary unless you have Gold so you can go online and download the gametype.
EDIT: I don't think you need Gold to download content from XBL. You do however need Gold to play the multiplayer..