~~~ I have had many chips, plaques and metal jetons from Menton... and your piece is unknown to me.

Most common monogrammed pieces during the period in question were "CCM" for Casino Municipal Menton or "CM" for Casino Mnicipal (w/ Menton mentioned on the obverse.

The casinos I know of (back then) were : Cercle des Etrangers, le Casino Municipal and Casino d'Menton and Villarey...

...and I'm also looking at monograms in "Jetons Metalliques des ercles et Casinos Francais et Coloniaux" and not finding your G.C.M. monogram.

Good luck and keep us posted if you find out anything.

Perhaps the "G.C.M. stands for something else... like Grand Cercle M.... or Grand Casino M... or Grand Casino Municipal...?(?)