I know this is a one year old post, but still, never know who might randonly come across it (eg. Me)...
I'm not a Linux user, so I can't help you there. But you're quite right about there being nothing on the Gainward site about these units, or any soundcard for that matter. Seems unusually strange, as Gainward isn't an uncommon soundcard brand. I can only assume one thing, which is I guess it's an entirely different company.
But yeah, I just bought one of these things, and from what I can figure the optical in or external power supply inputs don't work. I have however figured out that while the DC in is labelled centre positive, it's really centre negative.
I havn't tried full 5.1 yet, but this thing is starting to become a wee bit of a hassle! I just bought it for a cheap surround sound decoder (optical to 5.1 analogue), as $70 NZD is much nicer on the wallet than around $500 NZD for the real deal.
Hope this might help someone, somehow...