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