I would be very much surprised about that. Gras carbines in 1914 would be a weapon for reservists, and the cavalry did not have reserve units. The only possibility would be if newly raised squadrons had received those.
What is the source for that?
Anyway, from what I know, rechambered versions (8mm) were used for non-front-line troops and a lot were sent to Russia. Also the Greek army used it.
The French Cavalry would have been armed with the 3-clip Mousqueton Berthier, like the artillery.