When GarageBand launches up after being installed, it will prompt you to install extra content, which requires an admin password to install. If you're using Munki, you probably want to find a way around that admin password (either to not bother your users who are admins on their own machines or to not frustrate your users who are not admins on their own machines).
The Munki wiki has a great article on GarageBand (and the two main .pkg files that are required).
When you run the cwindus GarageBand content download script, you will see about 90+ other .pkg files in addition to the two required ones.
You may choose to ignore those. If you want to import them all into Munki, it would take a long time to manually import each one. Enter the little-publicized (which isn't mentioned on the munkiimport wiki entry, at least as of this writing) NOINTERACTIVE mode for munkiimport.
I wrote a little script as an example, which will recursively loop through a directory of your specification, find all the .pkg files in there, and then import them all into Munki noninteractively: BulkMunkiImport.
I wrote the script with the GarageBand content in mind. In theory, though, it shouldn't be too difficult to tweak for any large number of .pkg files you want to import into Munki. You may still have to do some cleanup afterwards using MunkiWebAdmin or MunkiAdmin, but it's still less work than you would have had to do without the NOINTERACTIVE mode.