Depends on what Garmin transponder it is. To have ADS-B out, you need the GTX-330 ES. Note, that's the ES version of the transponder.
We were all a bit misled when it came to Garmin transponders. The GTX-328 we were told didn't have ES/ADS-B capability whereas we were told the GTX-330 had. Turns out they want a lot more dosh to enable the 330 to be upgraded to ES. I think everyone in the UK with a GTX-330 should club together to form some sort of class action to get Garmin to upgrade our 330s for free as that's what we were sold in the first place.
Oh, you also say "to pick up Mode S traffic". That's not why you connect your transponder, you connect it to *output* Mode S ADS-B. The PilotAware picks up Mode S ADS-B traffic on its own.