GSM is Grenache Syrah Mourvedre. As you learned elsewhere there is a Cru Select kit.
However, I believe that Grenache & Mourvedre are the principal two grapes used in making Chateauneuf du Pape, with Syrah being another allowed/common grape in the blend.
CdP kits are usually called VCR (Vieux Chateau du Roi).
Steve