Wikipedia says that the MIME-type of the GPX files is
It is up to the document provider to determine the MIME types of the documents that it provides. For some document providers, they may know document MIME types from upstream sources (e.g., an email client that offers a document provider might take the MIME types of attachments from the MIME headers of the email). For everything else, they will tend to rely upon
MimeTypeMap, which in turn relies on an internal class named
MimeUtils has nothing for GPX, at least through Android 7.0.
So, you would need to take greenapps' suggestion, use
*/*, and be prepared to deal with parsing errors.
FWIW, I filed a feature request to get GPX added to