The Flashback Map Pack Add-on for Gears of War 2 has 3 achievements worth 50 gamerscore.
Complete waves 1 through 10 on all Flashback Map Pack maps in Horde (any difficulty)
Win a multiplayer match on each of the 5 Flashback Map Pack maps (any mode except Wingman)
Win a Wingman match on each of the 5 Flashback Map Pack maps