What recommend choice a navigator witch will be good with gpx tracks.
example: Like you create a map (with tracks) in Google maps, and then you save to kml format, next you convert to gpx, and upload to osmand or maps.me navigation, and then you can see all tracks, or if you have data, can use in maps.google, but maps google not work with gpx when a on drive function, and can't see road which in track. Otherwise osmand, when you drive you can see all tracks, and follow marked road, or on gpx tracks and get commands..

How work with that Garmin Montana 600 or 610?

Waits your recommendations