It depends, how you got the songs from iTunes.
You cannot use songs downloaded from Apple Music, but if you purchased song and albums individually from iTunes, you can use them.
For example, this song is showing "Protected AAC Audio File" as the file kind in iTunes.
It is one of the older songs, purchased from the iTunes Store, sold before Apple switched to selling only unprotected songs.
New iTunes purchases will show "Kind: Purchased AAC audio file" and you can use them.
If you ripped your songs from CDs, some CDs may have an embedded protection and cannot be used as well. But any song you recorded from the radio or older vinyl records will work with GarageBand.