Edit: As of Operation 5, there are now multiple reports this unlocks at ally Level 1.
The below is still accurate with the exception you only need to get to ally Level 1 to unlock this now.
Contrary to the 'buggy' flag, I don't believe this is bugged, the description is just a little misleading.
To unlock this achievement, you need to get a gears ally to level 3 (not level 1, as you might believe from the achievement description). A gears ally is simply another person you play Gears 5 with. Every time you play something with them, you get ally xp.
The needed xp as of Operation 4 is 400xp.
Level 1 - 25 ally xp
Level 2 - 125 ally xp (cumulative 150 xp)
Level 3 - 250 ally xp (cumulative 400 xp)
The fastest way to get xp is to play the Co-op vs. AI playlist (beginner) and then voting for Gridiron every time. You get 70xp per grid iron game.
Note: Ideally you should pair this with BFFs:
The BFFs achievement in Gears 5 worth 167 pointsComplete a Versus match with a team of four Level 5 Gears Allies