I should have asked this sooner, but does Gamersgate games support DLC not found on Gamersgate? There are a lot of DLC found on Bioware's website.
In the case of EA games, the GamersGate versions are basically the same as the retail/EA store versions. Thus, you can use the Bioware/EA store sold DLC with no problem. You can also mix and match with retail base games and GamersGate sold expansions too.
In the case of the Dawn of Discovery series, you cannot mix and match and you also have to make sure you have the right regional version. I.e., the versions sold on GamersGate are not compatible with US versions.
Games such as Borderlands (on the PC) cannot be used with any of the Borderlands DLC, etc., and this is generally mentioned on the store page.