Convert the MP3 file into a WAV using Media Converter (iOS) or Online-Convert (macOS and iOS).
To fix greyed-out files in GarageBand you need to convert the file either into a WAV file or a standardized MP3. Fixing this issue isn’t a problem at all though, and I’ll show you more than one method. One such example is GarageBand’s inability to process MP3 with an unsupported codec. Like any application though, there are some necessary workarounds for whenever you run into a limitation of some kind. This makes sense to me because it’s a solid, free, application that can create almost any kind of music you need it to. The iOS version of it (iPhone and iPads) appears to be growing in popularity too. GarageBand iOS/macOS is a great tool for creating music, and it can even be used for basic audio editing.