Mapy.com does that really well [0] (not affiliated). Not open source though. It uses mix of sources - OSM included - to render map (best render imo - especially the "outdoor" layer) and they are pretty large contributor to OSM in Europe. In free tier you can have one country downloaded for offline use. Sharing .gpx is no problem. Traffic data only in Central Europe.
reply[0]: https://mapy.com
Traffic info is for Central Europe plus favorite vacation destinations like Croatia and Italy. I don't know how reliable though.
replyI agree that the outdoor layer render is probably the best there is!
I tried a TON of hiking apps and this is the best free one hands down.
replyBeing able to download maps, tap to set destination, and see elevation all for free makes it top tier.