I don't now of any apps that can import/manage gpx routes, but CoPilot is probably your best bang-for-the-buck--full USA map (you can dl by state to save space), 1 year of live traffic, turn-by-turn navigation, 3d maps, offline POIs, for $10. The offline POI's are what sold me on this app--if I'm exploring a city it's always good to know what's around you, or where the nearest grocery shop is.
I've heard that if you convert the GPX files to TRP, CoPilot can read them. I'm not 100% sure though, as I can't even find on the app where to dl it. You could try contacting CoPilot support on that one.