If I buy the DLC from this site can I play the game on my home and at work computers through Steam or do I need to buy from Steam for Steam play to work?
You can mix and match steam and non steam stuff as much as you like, the game will not care. The only issue is to make sure when installing the game or expansions that it is all in the same place. Our installers do not know where steam put the game, so you have to tell them during install.
I think your answer may be a bit misleading. Based on this answer, Today I downloaded the Parasites from GamersGate- with the expectation that I could install on Steam (since it was mentioned the installer doesn’t care where it is put, tho Steam may have an issue apparently). There is no activation code on GamesGate to apply to a Steam installation. I then tried the Gamersgate installation into the Steam folder where GSB resides - it goes through the steps but in the end it won’t install and does not appear in the Steam game folder, the DLC list I own in Steam or ingame.
So it looks like I paid for the expansion and am unable to use.
Any fixes or suggestions, please.
Just tried this with the GSB Galactic Conquest add-on and it worked fine. Installed it (changing the path to the steam folder) but did have to manually move the data files as it had made a further gsb directory.
Might want to check that maybe it did the same thing with your add-on?