If you used Out-File, then it's not really an XML file at all, regardless of what file extension you gave it. 🙂 It'll just be a text file containing a table or list (whatever you'd have seen at the console if you hadn't piped to Out-File.)
Export-Clixml and Import-Clixml are paired commands; the import reads files created by the export.
You could have done this:
gps | Export-Clixml d:\process.xml
# and later: