I copied this text from the La Salette website after translating it into English
'It is also possible to get to the sanctuary on foot, like the old pilgrims, by following many marked paths, for example, from Valbonnais (north); Valgaudemar (south-east) or Corps (south-west).
All these routes allow you to discover the surroundings of the sanctuary, particularly rich in alpine flora and fauna, as well as to have some breathtaking views and admire the landscapes of medium and high mountains.'
I cant help you with the route to Corps, which would be heading in the right direction for the Via Domitia (GR653D)
https://www.gr-infos.com/en/gr653_D.htm but would say if you can get there it would be about 35km according to google earth to Gap, and then you are on the route to Avignon. My suggestion after that is stay on the route and go to Arles and take the Chemin Arles and follow it to Toulouse and take the connecting route up that goes to the Via Piamonte
https://www.caminodesantiago.me/com...-gr-chemin-toulouse-to-sb-de-comminges.49402/ .
I am being a bit vague but will try to find my book which does a Chemin St Jacques from Arles to Logrono via the Mediterranean and would cross the start of the Via Piamont.
+ If you couldn't find connecting trails between Corps and Gap then this is the road you would be walking
The Route Napoleon is a beautiful route leading from the south at Antibes / Cannes and via Grasse, Digne, Gap and La Mure to Grenoble.
villa-valbonne.nl
This link is to the tourism office of Corps, they have online map which you can move about and zoom in on, it may be a way of creating your own route.
Leisure activities in Corps: Find the right place to stay on your next trip with this wide range of quality establishments.
www.france-voyage.com