According to GPGs last statement, you can use your CD key and redeem a licence on Steam.
If you have a gold edition (Supreme Commander and Forged Alliance in one) you have two CD keys, and from my personal experience each of them can be redeemed for a gold edition on Steam. (-> so you could give one cd key to a friend 
)
Yes, you can link your account to gog.com after registration.
"Nerds have a really complicated relationship with change: Change is awesome when WE'RE the ones doing it. As soon as change is coming from outside of us it becomes untrustworthy and it threatens what we think of is the familiar."
ā Benno Rice
Gog is supported now. Therefore I deleted all prior discussions to avoid confusion.
"Nerds have a really complicated relationship with change: Change is awesome when WE'RE the ones doing it. As soon as change is coming from outside of us it becomes untrustworthy and it threatens what we think of is the familiar."
ā Benno Rice