You can now buy GMod with Team Fortress 2 on Steam. I suspect that most people reading this already own at least one of those so this is probably useless to you. But it's still news!

Regrettably, you won't be able to use any of the content from TF2 until the next version of GMod is released (Which is currently undergoing an open beta test on the forums). The release date for the next version is "when it's done" (aka "it isn't done yet, stop asking and let me work on it").