You need to leave at least three hours between your landing time and train departure.
It will take you, on average, an hour to deplane, go through immigration, claim your bags, walk through customs, and out into the airport.
It can take you up to another hour to get to the TGV station at the airport, depending on what terminal your flight lands at.
Somewhere in that time period, you're going to need/want to visit the toilet, get something to drink, and pick up food/drink to eat on the plane...and you need to build in time in case your flight is late and/or something is moving slowly.