Depends on how you upgraded to ML and how you acquired GB before hand.
GB is part of the iLife suite of apps and comes free when you buy a new Mac, you should have had a DVD with the apps in the box the Mac came in along with the OS DVD.
If you did a plain upgrade then GB should still be there, however, if you did a wipe and fresh install of ML then none of the iLife apps will be installed and you will have to dig out the iLife DVD or repurchase the apps from the Mac App Store.