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If you got Arma II free from GMG a few months ago you just need to buy Operation Arrowhead. Once you run ARma II and OA once it’ll make it Combined Arms.
Right now the cheapest place to get Operation Arrowhead is on Steam at $11.99 (40% off).
You can also get it from GMG for about $16 after voucher code if you want it on the same site. Finally, there’s Gamerocket, which has it for $12.49. It’s a German site so I’m not sure what kind of longevity they’ll have though.
Directions for getting DayZ to work with the GMG copy are here.
so the steam addon does work with a non steam version of the main game?