You can import GPX files into Goat Maps by selecting the "My Data" tab, tapping the + button in the upper right, then selecting "Import GPX File". This will show a file picker where you can choose a file to import.
If you have a GPX file in another application, using that applications export function should show Goat Maps as an option for opening the file.
A zip file containing many GPX files can be imported from the My Data tab in the same manner as a GPX file, but Goat Maps will not show as an option when exporting a zip file from another application.
Goat Maps does not currently support importing or exporting KML files.