It looks to me like France has two main GBM groups: (1) Hospital Pitie-Salpetriere in Paris, and (2) La Timone Hospital in Marseilles.

This page from Expertscape will help you find the individuals in those institutions who know the most about GBM.

http://expertscape.com/ex/glioblastoma/c/fr

The page is not perfect (e.g. the top person is at Harvard, not in France, and the second person ranks high because of a clinical trial), but it's probably the most efficient place to begin your research.