How can I buy a Lunea train tickets from Paris to St Gervais in February from the USA?
Go to www.tgv-europe.com. Put in Great Britain as your Ticket Collection Country. This will prevent the site from switching you to RailEurope should you put in USA. For TGV tickets, you can buy them online and print them out at home. For Regional-class trains, you can buy them online but you will have to pick up your tickets at any staffed train station when in France. No you don't have to go to Great Britain to get your tickets.
Do you mean St Gervais Chateaune or St-Gervais-les-Bains? One is south of Paris, the other south of Geneva.
What's your travel date? In February the direct night train only runs on weekends. If you mean to travel during the day, be aware that direct Lyria TGVs only run on weekends. On all other days of the week you will need to make a connection to a regional train at Bellegarde. For departure times requiring making that connection, you cannot print your own ticket because of the leg on a regional train. However, you could book Paris-Bellegarde on tgv-europe.com and print your ticket for that leg. Booking ASAP (up to three months allowed) can get you a cheap Prem's fare. Buy your Bellegarde to Saint-Gervais-les-Bains ticket in Paris. There is no discount for advance purchase tickets on regional trains and your ticket will be good on any of them.