Here is the full list of all 10 Kinect Sports Gems: Reaction Rally achievements worth 50 gamerscore.
Reach level 10 (or observe another player doing so) to gain the rank of Tennis Titan.
Reach level 9 (or observe another player doing so) to gain the rank of Red Flash.
Rapid fire! You reached level 8 (or observed another player doing so) to qualify as Yellow Flash.
No going back! You reached level 7 (or observed another player doing so) to rank as Return Champ.
Never miss! You reached level 6 (or observed another player doing so) to climb to Multiball Menace.
Looking up! You reached level 5 (or observed another player doing so) for promotion to Top Lobber.
Sharp work! You reached level 4 (or observed another player doing so) to rank as Backhand Slicer.
Wicked! You reached level 3 (or observed another player doing so) to advance to Forehand Fiend.
Imposing! You reached level 2 (or observed another player doing so), rising to Hard Court Heavy.
Good start! You reached level 1 (or observed another player doing so) to become a Racket Juggler.