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Garmins own program basecamp give you pretty much ultimate control over gpx tracks and routes, allowing you to split them, join them, add bits or take away bit etc etc etc. You can also move individual points on both tracks and routes. Most gpx tracks contain something like a 1000 or so trackpoints, therefore you can adjust the track in something like one metre increments. This is most likely OTT for your purposes but you get my drift hopefully.
I see searched the thread but could not find this: what is the go to editor for gpx files? I am doing some touring soon based on some of the lost lanes stuff and want to add in some detours and directions to camp sites, etc.