GPX Smart will take your GPX trace and fit it against routes that Valhalla component in OSM Scout Server knows. This is used to allow you to get navigation instructions (like "turn left on the northern street").

For it to work, you'll need:

* osm scout server with the maps for the area in question
* correct transport mode selected in gpx smart settings

If, OSM doesn't have any roads that can be fitted with GPX, or, if you want to go by car and the street is unidirectional in opposite direction of your movement, or some similar issue, it will not work.