Per Bike Calculator, a rider doing 3 W/kg on a 7% grade would be doing slightly under 8mph. Using HTML5 Gear Calculator we can see that this rider could go up the hill using a 34-28 at ~77rpms, or using a 34-25 at ~70rpms or a 39-25 at ~60rpms or a 34-32 at ~90rpms or... I think you get the idea. Which of those is best is entirely up to the rider.