I see the same error, it started recently for me. I don’t know PHP, but I had a look at the file and this is what worked for me:
– make a backup of wp-gpx-maps.php
– remove $avg_cad from the list on line 300
– remove line 336 ($avg_cad = $cache_obj["avg_cad"];)
As I said, I’m not familiar with PHP or the plugin code (it seems trying to include a variable in the cache file name that doesn’t exist yet) so use at your own risk, but this removes the error message for me and doesn’t seem to affect anything else.