I don't think you can.
On Garmin Nuvi's you have the ability to upload the GPX files, as you've done, but there is no simple upload means for the built in Garmin.
Also note the built in Garmin has no traffic in the UK so you may wish to employ Google Maps via Android Auto.
I haven't examined Google Maps for saved routes that have been 'via' or 'avoid' changed but there may be a way.