You cannot load tracks on Garmin's automotive devices, this has always been the case and is a common complaint. So I assume that your "maps" are actually tracks, and those just won't ever work. You can load gpx files with waypoints, but not tracks. To clarify: gpx files can contain either lines or points. Lines are called "tracks" and points are called "waypoints". There are also "routes", but on a Garmin Automotive Device, these will be converted to "trips".
Apparently Garmin has done this intentionally, because there's no technical reason why it shouldn't work. But Garmin has always had a strategy of removing certain features from devices to force you to buy something more expensive. In this case, they want you to buy an outdoor-type device if you need to use tracks.
Now there is a work-around, but not very simple. You can use one of these programs to turn your tracks into transparent maps that will display on top of the City Navigator map. The first one is free, I have not used either.
http://www.javawa.nl/imgfromgpx_en.htmlhttps://www.gpx2img.com