It'll be offered in two packages. Due to the mod requiring a Source engine-based product to run, Valve's offering it bundled with Counter-Strike: Source for $24.95. Or, if you already own a Source engine-based product, you can buy it for $9.95.
The mod has found popularity due to the inclusion of a physics-based sandbox feature that lets you do wacky and crazy stuff with Source's physics, along with mini-games and the ability to create your very own games.
Garry's Mod creator Garry Newman informs that the new version of his project "has been created from scratch so almost everything has been revised and re-invented. It has a bunch of new tools to play with (winches, hydraulics and hoverballs), totally new spawn menu and a few new Post Processing effects to play with.
"The great thing about Steam is that I can update really regularly, really easily - which is something I plan on doing - so expect more tools/gamemodes/stuff to come."