I’m guessing you really don’t have adsb-in. You do have a mode S transponder, probably, that can pick up TIS (traffic information service) when in the right kind of coverage. The FAA is phasing this out so it’s not surprising if you only get it near a major airport where it still exists.
Since your header says you’re in San Diego, you do know that adsb-out is required within 30nm of KSAN? Or above 10,000’? Also, there are some circumstances where you will not get traffic info without ADSB-out?
To answer your specific question: to get ADSB-in, and display it on your G900 screen, you need an adsb-in receiver from Garmin, or another manufacturer that can interface with the 900. Since Garmin keeps the required software proprietary the latter is hard to find.