Here is the full list of all 12 Orbital Gear achievements.
Kill 500 Gears
Get the most assists in a match
Kill two Gears without landing
Destroy 100 buildings in Orbital Warfare
Stop 50 projectiles with shields
Shoot down 10 projectiles
Knock an opponent out of the stage
Kill 10 Gears with Mines
Get the most kills in a match
Hit two players with the same Gauntlet attack
Hit two players with the same Discharger shot
Kill the same player three times in a row