If you clicked the download link for a GPX file, you will be asked whether the GPX file should be loaded (step A). If you use the default Safari browser on the iPhone, the file is stored in the download folder, which is made visible by the symbol in step B. Clicking the icon displays the downloaded file (step C).
By clicking on the file, the rather cryptic GPX file is displayed in text format. By clicking on the forwarding symbol at the top right (step D) you get to the dialog where you can choose which app you want to use to open the file. "Outdooractive" may be hidden behind the item "More" (Step E). If you got to your outdoor app in step F, you're almost done! Then the corresponding app should open, you have to confirm the import of the file again there (step G):
PS: If you send the GPX files to your smartphone as an e-mail attachment, you go into the process in step D. This works quite well for most Android smartphones (and of course slightly modified compared to the screenshots above).
27 October 2023
Unfortunately for me the gpx file is always opened with another app installed (Sigma Ride), which can also process and display the data correctly. How can I set iOS to generally open gpx files with Outdoor Active?