I haven't done any research for the Gagne line in France (yet), however, I did find the Father Gagnier web site. He has been to France and took photographs of where Louis Gagne of the church he was baptized in; was married in; the church where he married Marie Mitchel and the mill where he worked in France. Along with the translation of his marriage record and more.
I have found a number of Gagne records in the Drouin collection, even one for Marie Mitchel, I believe.
Another descendant of Louis and Marie, has contacted me. Sabrina, descends from their son Pierre. She only had her Gagne descendcy.
I would love to share what information that I have. I'm related to the Gagne line through my Allain line.