If GB is not telling you to download additional content when you go to select certain loops then you have already installed them somehow... It tells you when you select a greyed out loop in the loop browser.
Frankly, though it's a pain in the butt...what I would recommend is the following.. which is start again from scratch.
Make sure you have all your personal projects and other data such as docs saved...
Then start up your Mac holding down option and R (This is to put the Mac in Recovery mode...)
Here is the support doc for Recovery Mode if you are unsure about anything...
http://support.apple.com/kb/HT4718
Use the Disk utility to erase the HD
Install Mountain Lion on that fresh drive
Go through the setup for ML using your Apple ID and Password when asked...
Install/.Run GB from the App Store under the purchased tab... (You won't have to buy it or Logic again..)
Open the loop browser in GB and select a greyed out loop (if there are any.. in some circumstances all GB content is installed during initial setup/installation of GB) and it should ask you if you wish to download the additional content.. Do so.
Now restart your Mac (Important step) and wait about 20 minutes for Spotlight to finish reindexing your system drive (equally important!) You can check if spotlight is done by moving your mouse pointer over spotlight's icon in the menu bar and it will tell you it's current status...
Now go to the App Store and under purchased tab select and reinstall Logic Pro
Once installed run Logic and go to the Logic Pro Menu bar and under Logic pro select Download Additional Content and select ALL Content...
Wait.................
Once that is all downloaded and installed... quit Logic and restart your Mac.. and wait again for about 30 minutes for Spotlight to finish it's indexing of all the new content... Do not start Logic again until after this rebuild has finished...
Now install any 3rd party plugins and libraries...
Start Logic in 32bit mode to validate any 32bit plugins you may have...
Quit Logic...
Start Logic in 64bit mode... and you should be set to go!
Note: It is really important to follow these steps in exactly in this order without missing any out.. to ensure the best results I have found.
Good luck!