I happen to be taking the same TGV line (Paris -> Nantes ) this week-end during the strike!
First and foremost, you should double-check that your train is effectively cancelled (if so, you must have received a mail from the SNCF).
If you did not receive anything, you're good ! Your train departure will probably be fine.
If you've received a mail notification that your train have been cancelled, SNCF will offer you to reimburse your train ticket, or to exchange it free of charge for an other day.
The later seems like the best option for you, it may mess up with your schedule a bit but it is worth considering.
One point worth mentioning is how will you go to Gare Montparnasse ?
You should note that except for line 1 and line 14 which are working normally, every other line encounter issues.
Please see the annoucements by RATP (Paris metro lines) : https://twitter.com/ClientsRATP
At the moment, the traffic is:
- Metro 1 and 14 are working fine
- Metro 4,7,8,9 are perturbed, working only between 6h30-9h00 and 16h30-19h
- Metro 2,3,5,6,7,10,11,12,13 are not working
- RER are working between 6h30-9h00 and 16h30-19h
Since you have to go to Gare Montparnasse to get the train to Nantes, your best bet is to use line 4.
Also use line 1 and 14 as much as possible!
If for any reason you do not want to take the train, then you could use the bus which will certainly be cheaper than a taxi ride. Some bus company are BlaBlaBus/Ouibus, FlixBus, Eurolines...
Finally there are a lot of electrical scooter in the street in Paris, or bike that you can rent (Velib') to go to the station.