Gare de Lyon is closed that weekend for engineering works. Capacity is limited and some TGVs are departing from different Paris stations.
For example the 14:42 departure instead leaves at 14:05 from Paris-Montparnasse. It shows as fully booked but this is not true, this is a SNCF tactic to prevent domestic passengers onboard this train (which continues to Barcelona).
So you’ve got 2 options:
- book the 14:05 as far as Barcelona (and get off in Nîmes, no problem with that). Though reservations will be 35€ instead of 20€
- take the 16:13 from Paris-Montparnasse to Nîmes Pont du Gard
More trains might be released soon (i.e. there are no trains to Marseille for now) but I don’t know the details.
EDIT: you could also try one of the Versailles - Lyon TGV + Lyon - Nîmes TGV. They stop in Pont de Rungis which is on metro line 14 which would allow a swift connection but I can’t seem to book them either on SNCF (for normal tickets) or Rail Europe (for Interrail reservations). Only from Versailles works. So might be worth calling SNCF if you go that way.
For example:
- TGV Pont de Rungis/Aéroport d’Orly - Lyon Part-Dieu 15:01 - 16:56
- TGV Lyon Part-Dieu - Nîmes Centre 17:10 - 18:41 (or 1 hour later)