Doofuwheam86 Hello, no, your ticket is not valid in the RER, but you can buy the tickets in advance at the bar in the TGV Lyria from Geneva and avoid long waiting time at the counter offices or vending machines in Paris.
Online tickets could also be available with Android phones, but I never tested this. It doesn’t work with iPhones, you need a « Navigo » pass to load the tickets. You can open the gates with your phone, your pass or the paper tickets with a magnetic stripe. Some gates don’t accept paper tickets.
In the RER you also need the ticket at the destination to leave the station.