how to adjust the pitch of an audio loop ?

Do you want to create a chord progression?

You can transpose regions in a track, but the transposition will apply to the complete song section, so you will have to create plenty of song sections.

See this Help page:

http://help.apple.com/garageband/ipad/2.0/index.html#chs175dc411

Transpose regions in the track

• You can change the pitch of (or transpose) recordings or loops in a track up or down by semitones or octaves. After you turn on transposition for a track, any new recordings or loops you add to the track are also transposed.
• Tap Transposition.
• To transpose by octaves, tap the Octaves up or down arrow, or swipe vertically to transpose by multiple octaves.
• To transpose by semitones, tap the Semitones up or down arrow, or swipe vertically to transpose by multiple semitones.

Transposition applies to all regions in the track for the current song section, but you can choose different transposition values for the same track for different song sections. If you move or copy a region to a different section with a different transposition value, the value appears as Multiple. For more information about song sections and transposition, see Extend a song with song sections.

Have you clicked the Mixer button in the control bar?