There currently are a variety of options to travel by night train from Gap (France) to Paris (France), taking between approximately 9 and 12 hours: Namely the INTERCITES DE NUIT n° 5792, the 770A 64002, the 770A 64006 and the INTERCITES DE NUIT n° 5790. These lines are operated by SNCF.
The National Company of the French Railways (SNCF) operates a handful of lines throughout continental France.
In their night trains, they typically offer two types of carriages: seats and couchettes, with the latter available in first and second class.
Couchette carriages generally have 4-person compartments in first class and 6-person compartments in second class. The berths are simply designed in both classes and are about 1.9 meters long.
Yes, they are available. You can book an entire couchette compartment for private use. This can also be done for a group.
The main difference is in occupancy: four people per compartment in first class versus six in second class. Additionally, first class berths are slightly wider. First class passengers have access to showers at stations in Paris and Toulouse. Second class offers bicycle transport for 10 euros, which may interest cycling tourists.
The night trains don't have a dining car, so it's advisable to bring your own food or eat before departure. You can also pre-order salads, sandwiches, and drinks. First class passengers receive a small breakfast. All travelers get a night kit with refreshing towelette, dental hygiene product, sleep mask, earplugs, and a bottle of mineral water.