Can you specifiy the date and details of the trains you’re looking at? Maybe the train you’re looking at from GDL to Antibes is (partly) a OuiGo, that’s indeed not included. But there should be plenty of other connections via ordinary TGV+TER.
Check on this website https://travel.b-europe.com/Eurail-GE/en/booking-tgv#TravelWish to get your seat reservations (for which you need a pass cover number). And maybe split up your journey (Pontorson-Paris and Paris-Antibes).
This is surely because it is a Ouigo train (low-cost TGV). Passes are not valid on them.
There are other TGVs on the route though so it won't be a problem.
Keep in mind that you'll need to cross Paris so count 1h-1h30 margin :)
- Pontorson - Rennes 08:42 - 09:33
- TGV Rennes - Paris-Montparnasse 11:35 - 13:19
- subway transfer
- TGV Paris Gare de Lyon - Antibes 15:17 - 20:40