If upon attempting to upload a GPS file you see an error message telling you that your file is "Not an activity," this means that your file is incompatible for upload to Strava.
The problem is that your file is likely saved as a "Course" instead of an "Activity" file. These files most often come from Garmin devices - the Course files are used for navigation and Virtual Partner™ purposes, and can be saved in either .TCX or .FIT format.
Because these files do not necessarily reflect the recording of a real-world activity, they will be rejected upon upload. If you have any additional questions please proceed with creating a support ticket.