Arriving at CDG airport and need to get train out of Gare Montparnasse to Brittnay. What is the best way?
One way is the Air France bus "Les Cars". You do not have to fly Air France to use the bus. If you check the website, you'll see there's a route that runs from CDG to Montparnasse--the bus drops off/picks up at the train station. This is one way that's pretty painless with luggage. You can buy a ticket in advance, at the airport or from the driver.
I'm sure others will weigh in with their thoughts on other options.
This link is to a similar question on the travel forum in 2014 of a traveler needing to get from CDG to Gare Montparnasse to then continue on as you are doing. You can also find good information by Googling 'CDG to Gare Montparnasse'.
https://community.ricksteves.com/travel-forum/france/cdg-airport-to-montparnasse-train-station
Taking Les Cars is the simplest most direct route. Taking the RER into Paris necessitates either a long walk or transferring to the metro followed by another walk. After being on an international flight, keep it simple. The fare is 17.50€ and well worth the convenience.