Here is the full list of all 12 Gotham City Impostors (GFWL) achievements worth 1,000 gamerscore.
Kill 50 enemy players using the panic-slap melee attack.
Accept your first Promotion.
Complete all feats related to a particular gadget or support item.
Earn a 10-kill streak.
Finish one match of Gotham City Impostors.
Unlock any locked loadout or costume item.
Complete all feats related to a particular loadout weapon.
Turn the tables in Fumigation when the enemy team's victory is imminent.
Win each game mode in Ace Chemical, Amusement Mile, Crime Alley, Gotham Power and The Docks.
Complete the Initiation mission.
Earn 50 gold medals in Challenge mode.
Become a Nemesis 50 times.