I can't imagine how the average grade for something like a city would be useful. Nor do I know where that might be published. Averages might never be encountered.
Like others, I just suggest you look at the possible routes you might want to ride on RideWithGPS, Strava, Garmin Connect, MapMyRide and many other places that let you create your own routes. They'll show you both the average grade of that route and the maximum grade you'll encounter. And there'll be a elevation profile of the ride that you can look at to show you everywhere that you might have an easy time pedaling or a hard time pedaling.
If you don't know where to ride in those places, then they also have heat maps that show you where others ride. Or you can just browse routes and rides of others that have made them public.