Hi Geir,
To get the best of Garageband's sounds, you should set the velocity to a fixed value of 100 or 127 (in the Settings menu, in the MIDI Mappings section). As Garageband translates the breath intensity values sent by the Sylphyo to volume changes, setting a fixed velocity allows you to benefit from the maximum sound amplitude (it'll sound less "tiny").
However, while compatible, the sounds of Garageband aren't made specifically for wind controllers, so it's likely that you'll find them a bit lifeless anyway. Our user guide has a list of other virtual instruments that you might want to try out.
In addition, if you own Reaktor and acoustic instrument simulations are your thing, you also might want to get the free Silverwood and Silverwood Bb Clarinets patches (the last one sounds excellent).