If you import GPX files successfully (i.e., you see a green checkmark), but cannot find the track, there are two common reasons:
Hidden by date filter
The dashboard (first screen) and track lists have a date filter that is set to the last two weeks by default. Imported files often have very old timestamps and are therefore hidden by the filter. Tap the filter at the top of the screen to adjust it.
Waypoints instead of Tracks
Some GPX files do not contain tracks, but just a large set of waypoints. Therefore, you will need to look in the waypoints section to find them.
You can use gpsies.com to convert waypoints to tracks.
Imported tracks are grouped in a separate section at the bottom of the dashboard. Imported tracks are not considered for your summary statistics on the dashboard and are not exported to Apple Health. If you’d like to change that, open an imported track and tap its title. You’ll be able to remove the import tag on that screen.
If GPX files fail to import (red cross instead of green checkmark), it is likely because of invalid or missing timestamps. In most cases, you can use gpsies.com to convert those files to valid GPX files.