If you try to upload a GPS file and see an error message that your file is "Not an Activity," it's incompatible with Strava.
This happens when your file is saved as a "Course" instead of an "Activity" file. These files most often come from Garmin devices — Course files are used for navigation and Virtual Partner purposes and can be saved in either .TCX or .FIT format. Since these files don't necessarily reflect a real-world activity recording, Strava rejects them on upload.