Just some Swiss guy that loves to do maps, flags, and languages. Don't you adore when just one picture narrate a whole history? …Not to say that a bunch of pictures can't work beautifully together as well.
From what I have studied from Romance linguistics in Switzerland, sadly, Gaulish was most probably lost in Helvetia and Sapaudia by the 5th century. Helvetia was heavily romanised early on, being on commercial ways on the Rhone and through the alps, it probably started before the Roman conquest even with Gaulish aristocrats learning latin for commercial and cultural reasons.
aAlso, I believe the penetration of Allemanic was more to the centre than the east like depicted here. The first areas to Germanize were more along towards the Upper Rhine from Lake Constancand and maybe a bit into Argau. The region you depicted corresponds too much to the original area where Romanch was originally spoken where Germanization was slow and really started only with the Walser Settlers in the 10th century.