navigation apps which work well for hiking/cycling and allow to open and use external GPX files for navigation.
from a cycling perspective: Organicmaps/Comaps can import a folder with gpx files, but not yet route along an import (1, 2). Their routing engines are not as flexible as brouter / bikerouter and there’s no interop (1, 2).
OsmAnd can do both, follow an imported route and use brouter. Its v2 map layer rendering with vectors isn’t efficient (3) though. v1 + online bitmaps is quicker. Its pro and con is its many knobs.
If you have a bike computer, Gadgetbridge can be used to transfer gpx to some Garmins.
komoot is good for planning, but while the service itself relies on commons (OSM) and rider contributions (tracks, heatmap), it behaves like a data silo for the latter. You can export gpx, but need the region or a world package - and it’s unsaid if bendingspoons will honor that indefinitely.