I was looking this up at the same time, and the INAO rules say at least 80% grenache, with a minimum of 15% syrah and mourvedre combined. Here’s the INAO requirement (under the “Reconnaissance” tab):
Mode de production
Encépagement :
grenache noir limité à 80 %. syrah et mourvèdre minimum 15 %.
autres cépages Côtes du Rhône, excepté le carignan, limités à 10 %.
I don’t know why that’s inconsistent with the appellation rules you found.
In general, the grape % rules in the Southern Rhone are for the vineyards, not the wines, so it’s possible to have a 100% Syrah, or even a 100% Mourvedre, from Gigondas. Unquestionably, the majority of the wines are mostly Grenache.
As to this wine, Louis’s base Gigondas is a pretty classic blend and is always roughly 70% Grenache and 15% each of Mourvèdre and Syrah… there might be some Cinsault in there as well, I believe… however, his three single vineyards are 100% Grenache. All of the wines are not destemmed and aged mostly in used Burgundy barrels, and it has been that way since 1992 when Louis took over from his parents, and I expect even before that… These are always rich, powerful wines that need bottle age… I always have to schedule my visit to Saint Cosme at the end of the day as the tannins in his young wines are brutal… they do integrate, however, and the wines are what they are. I personally love them. In addition, 2019 in the Southern Rhone is a big, powerful vintage (think 2016, 2010, 2005 maybe, very close in style to 1998), so if you want Pinot Noir like Grenache right out of the gate, I’d look elsewhere.