If upon attempting to upload a GPS file you see an error message telling you that your file has "Improperly formatted data," this means that your file is incompatible for upload to Strava
There are many things that can cause this, but essentially the issue is that the data in your file has either been corrupted, is missing information, or is improperly formatted.
Strava accepts files that conform to the .FIT, .GPX, and .TCX formats. These formats can be very sensitive to the exact syntax of their data; meaning that a single stray punctuation mark or an opening bracket that is not properly closed can cause the file to generate this error upon upload.
We often see these files come from Garmin devices if the device has experienced a recording error, or from 3rd party tools used to manually create or convert GPX files. If your device records a poorly formatted file, it can sometimes help to perform a hard reset on the device - or to contact the manufacturer. If you have any additional questions please proceed with creating a support ticket.