Is it an audio loop or a software instrument loop?
For an Audio Loop, search the GarageBand Help for "Transpose Audio Regions":
You can transpose audio regions in the Audio Editor. Audio regions can be transposed up or down by up to 12 semitones. A semitone is the smallest distance between two musical notes. Notes with the same name repeat every 12 semitones, called an octave.
Transpose selected audio regions
- In the Tracks area, select the regions you want to correct.
- Make sure the Region tab is selected in the Audio Editor inspector.
- Drag the Transpose slider left or right to transpose the regions in semitones.

For Midi Regions see this Help section:
You can change the pitch, or transpose, notes in MIDI regions to a different key. This is especially useful when you want to reuse a melody, rhythm part, or other pattern of notes, at a different pitch.
Transpose selected MIDI regions
- In the Tracks area, select the regions you want to correct.
- Make sure the Region tab is selected in the Piano Roll Editor inspector.
- Drag the Transpose slider left or right to transpose the regions in semitones
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Léonie
P.S. Your Question says GarageBand '11, but I am assuming you are asking about the newest version, that came with Mavericks, that is GarageBand 10, and that is what my post is showing. In the latest GarageBand '11 you can still use the transpose sllder as always.