Gene, 30, has been employed in the same role with BMW-powered Williams for the past four years with three race appearances during that period as a stand-in for Ralf Schumacher.
The Catalan's move reunites him with his former Minardi teammate Luca Badoer, Ferrari's official reserve driver. The two raced together for the small Faenza-based team in 1999.
Nine of the 10 Formula One teams, including Williams, have agreed to halve the amount of testing they do during the season next year, reducing it to 24 days.
Ferrari, who have their own private test track, have not signed up for the agreement.
