We've encountered this reoccurring issue on Apple devices sometimes. It seems Macs don't always recognize .gpx file extensions as being .xml files, and thus add an extra extension.
You can fix this by modifying the file extension.
In Finder, locate the file. Right-click and select "get info".
Find the section "Names and Extensions". Delete the .xml extension so that your file is only a .gpx.
Select "use .gpx" when prompted.
Your file is now compatible with Gaia GPS.