Hi Linnea,
From Gillonnay to Le Puy following the GR65 it's 180 km. If you take the detour to Saint Anthony it's 211 km, so basicaly 30 km more. And yes, the detour follows coupled of GRs.
I cannot tell exactly how many days it will add and I didn't follow exactly the stage of the Green guide as I carried my tent with me, but there's at least 6 days when you'd have to walk 24 km.
But this depends : do you want to carry your food and prepare your diner in hostels (gites) or do you want to stay in B & B's (or hotel) with a half-board (diner + breakfast) ?
If you have a map you can check by yourself, from Gillonay the way goes to :
Roybon, the Abbey of Saint Anthony, Romans s/Isere, Glun, Saint-Péray, Alboussière, Chalencon, Saint-Agrève, Saint-Front and Le Puy.
But the Abbey of Saint Anthony and the view (and quietness) will compensate this extra effort :wink:
Hope that helps and let me know if you need for infos !
Laurent