GPX file download

How can I download a GPX file or KML file from for example https://www.japan-guide.com/map/?id=nikko_full. This is so I can use it on my phone to walk with the route :) Thanks for your help!

Why are you asking a technical question on a Japanese forum instead of a tech forum or just using google to find your answer?

Well, looking at the source for that map, the markers are not GPX files (or KML), but hard coded grid references. For example:
window.Itinerary_Gmap.markers = [ {"color":"","x":"36.75333999975067","y":"139.6040082448793","title":"Shinkyo Bridge","img":"\/thumb\/XYZeXYZe3814_375.jpg","url":"\/e\/e3814.html"},{"color":"","x":"36.75511639359675","y":"139.60046097552708","title":"Rinnoji Sanbutsudo","img":"\/thumb\/XYZeXYZe3803_375.jpg","url":"\/e\/e3803.html"},{"color":"","x":"36.75424927248889","y":"139.60065677678517","title":"Shoyoen Garden","img":"\/g19\/nikko_full_shoyoen.jpg","url":"\/e\/e3803.html"},

It's quite straight forward. First you need to harvest the data from the source.
Paste it into Excel, clean up the data and save as a .csv so you have the latitude and longitude in their correct respective columns.
Next import the .csv into a GIS like ArcGIS or QGIS.
Now you can export to .kml or .gpx to import into your device of choice.

Or you could just bookmark the page.