As isteveus said - the loop browser in GB has its own categories.
If you are not happy with the way Apple indexed a loop, you can save a new copy (Edit > Add to Loop Library) and pick your own tags or change the name to make it more descriptive.
To have a database of your finished GarageBand songs, share the finished songs to iTunes - this will give you all kinds of search criteria to retrieve the songs.
But you are right - there is really no database to retrieve GarageBand projects. I use essentially the Finder and a set of folders to structure my hundreds of finished and unfinished projects, plus Filemaker '11 to remember where I have the sheet music, midi files, song lyrics, etc. But I do not know of any ready made Filemaker template. I simply used "define database" to set up my own database view.
Send feedback to Apple, to let them know that you want GarageBand to have a project database like the other iLife applications: http://www.apple.com/feedback/garageband.html
Regards
Léonie