The description says everything and it's retroactive ( Credits go to:Homunculus Fury), that means if you have unlocked the cheevo you will get it on the main menu
If you are new on gears 4 thanks for the games with gold, you can complete the campaign on split-screen or play online, I recommend playing the difficult you want or choose the insane difficult with a partner on casual, both of you will get the achievements related for difficult (casual, normal, hardcore
& insane) and a cool character as Gilded Kait
By the way, you can unlock a cool set of skins called "Gilded", you just need to unlock one achievement on these games:
Gears of War or Gears of War (PC)
Gears of War 2
Gears of War 3
Gears of War: Judgment
Gears of War: Ultimate Edition (Xbox o PC)
Gears of War 4