Unfortunately your Nuvi 50 does not support either Routes or Trips.
The Best you can do is have multiple POI made From Your GPX Files and navigate to each in turn.
Alternatively If you have the skills you can Make an Overlay Transparent Map from your GPX Files With the Track in a Bright Colour and then Follow the Track.
Have a look in the Map Making Section at this Software, It is quite easy to use.
[Only registered and activated users can see links. ]