I think that you meant train from CdG to
Massy TGV train station?
@SallyJane, you have the choice between the two options described: from CdG through Paris first by RER train and then by metro to Gare Montparnasse, or from CdG by TGV train to Massy TGV station, avoiding crossing through Paris.
I'd just like to add one thing: journey time from CdG to Montparnasse is indeed around 60 minutes. However, I remember that the first time that I did this or a similar trip, it took me longer than the second and subsequent times, because the first time I needed longer to figure out how and where to buy train and metro tickets in Paris, which stairs and corridors and directions I had to take in the various stations and so on because everything was unfamiliar. However, I cannot say how much this added to my journey time through Paris.