Doesn't look like Steam is offering :Queens Gambit as an expansion anymore, but if it has to be through Steam -and you're not concerned about paying for Arma:Combat Operations again, you can pick up the bundle ArmA:Gold
...which contains both titles.
Or you could always pick it up the Sprocket version via this sites front pages link
-funny what a seconds worth of searching turns up
- through BIS' online store