I want something that should (I think) be relatively simple but can’t really find it.

I have “GPX viewer” on my phone but that uses google maps. Great when I’m on good wifi but not so good when I am on data or have no connection.

Is there something similar that would let me download OSM maps to use offline. Then put a gpx file on over the top? I can do it on my garmin – it is my standard method when road cycling in Europe. The map files aren’t massive so fit comfortably on an SD card with room to spare.

Would be nice to use as a backup to my garmin and for navigating when I don’t have data.

I was hoping a root around GPX Viewer would let me use an offline map but it doesn’t seem to be the case.

Galileo maps is offline OSM and let’s you import GPX. Not sure if it routes though.

Maps.me is same, offline OSM, it does do routing (directions not routes) but not compatible with GPX, it will open KML files, but you’ll have to convert the gpx to kml first!!

OsmAnd can do that. Though somewhat confusing interface – I think its usually quicker and easier just to use Maps.Me.

Viewranger.

or Back Country Navigator (I use Viewranger)

Alpinequest does it fine. I have been using it like that for years. As a bonus it still allows you to read the old Memorymap 1:25K qct maps from your PC/SD card so yopu can navigate in the UK with only a gps signal. You can also store satgellite images and overlay them with the OSM or OS maps

Backcountrynavigator… Paid for app (but not very much money) and you can load gpx in as a track