Retail game and gamersgate DLC……

[Slayer]

As with many games even though it is the same game, you can't mix n match retail, steam, gamersgate and other online service variations. Especially steam because they use their own copy protection. I did once buy Private military company for Arma from GG and it worked with the retail store box version but steam is a definate no go.

Best policy is to just buy your DLC from where you got your original, just watch for sales , they all have them..