(Will be updated whenever we stumble across a good site)
https://www.routeyou.com/en-gb/route/search/0/outdoor-route-search (Mostly European)
http://www.reisroutes.be/stadswandelroutes/alleroutes-3.aspx (Only in Dutch – Excellent walks though)
http://www.mapmywalk.com (Companion site to the app with the same name – registration required)
And of course, you can perform an internet search. If you want to walk in Paris, then search for “gpx paris walk”. Even better, perform your search in the local language : “gpx paris promenade” gives more usable hits than “gpx paris walk” .