Applies to: Launchkey MK2, Launchkey Mini MK2
Garageband doesn't natively support Control Surfaces, so the full functionality of the Launchkey won't be available with it.
In Garageband, you can use the Keyboard to play your Virtual Instruments, and the Faders and Knobs will output MIDI CC Messages (detailed here) for plugins that support Manual MIDI Mapping. Additionally, the Pads will also send out fixed Note Data.
However, in Garageband, you won't have Transport, Volume or Pan Control on the Launchkey itself. For that, you would need a DAW that supports the HUI Protocol (some of which are detailed here), or Ableton Live, where it will work to its fullest capacity.