angstrom: I wonder if Gamersgate is less restrictive and I can mix and match expansions if they have a sale.
They don't support versions sold by other retailers. Retail copies are only supported when publishers allow it (such as with the Dragon Age expansion as GG's version is basically identical to the retail one).
But generally, if you want an expansion of a game from a certain service, you'll need the base game on that service too.
I know it sucks, but generally, the problem is down to how the different services wrap the products which makes them incompatible with one another. However, there's no harm in asking support [at] gamersgate [dot] com. The worst they'll do is say that it's not possible.
Now, for some good news, if you happen to buy a product on GamersGate only for it to have a price drop that day, they will refund you the difference (in blue coins) if you ask them. I know this, as that exact situation arose when they lowered their pricing on VVVVVV to match that on Steam. Someone had bought it earlier that day for the higher price and was offered the difference in blue coins.
If that had been Valve, they would have simply said "tough luck". Although I do wonder if they'll stick to that stance once this "Steam Wallet" rolls out, as if they really wanted to improve customer relations, they'd see the benefit in using it to help offer partial refunds in similar cases...
