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