I'd get my feet wet first riding Gabrielino Trail next to JPL that goes to the Brown Mountain Dam waterfall. It's not very steep and with your gearing it'll be a nice trail to master. Once you start building your climbing legs you can try riding Fern Truck Trail. I'd advice against trying El Prieto Trail loop. It's doable on a gravel bike, but a bit difficult, you'll have to carry your bike on certain sections and it's bumpy as heck with no suspension. Verdugo Mountain is another uphill grind, but once you get to the top it's pretty easy going from one end to the other. Also right now it's really hot during the day, either ride late afternoon or carry lots of water. I easily can go through my two water bottles on uphill climbs, so stay hydrated. I also ride the fire roads up Griffith Observatory. Lots of roadies trying to beat their Strava times, I always get my butt kicked there on the uphill. The downhill is fun especially down Vista Del Valle Dr. fire road. It's better on a gravel bike since there's lots of loose rocks and dirt on the way down.
Most the trails in the area are an uphill grind, so I have a 2nd wheelset with slicks when I just want to take it easy. Saying that, I'd rather have quick access to uphill trails and fire roads, than be stuck on the flats!