from http://www.logicprohelp.com/viewtopic.php?t=10978

1) Create an audio track of the desired rhythm – you can do this by entering notes into one of the MIDI editors and use one of the instruments to play it, a piano sound will do. Then bounce it down into an audio file and import it into the desired track – track 1 for example.

2) Insert the noise gate on the track you want to affect and select the sidechain input as track 1.

3) Turn on the gate’s sidechain mode and adjust the threshold, attack, hold and release parameters to taste.

4) Mute track 1 (unless you want to hear a piano going ‘da da, da da, da daa…’ throughout your song).

Job done.