I've got a retail copy of Warband, and can't seem to reclaim it. I redeemed it on Steam years ago, but Steams lists it as a different key altogether.

Only the russian version of M&B can be redeemed. Gog is quite clear on that matter:

[i]Which game keys can you reclaim?

Editions sold worldwide:

S.T.A.L.K.E.R.: Call of Pripyat (GameSpy)
S.T.A.L.K.E.R.: Shadow of Chernobyl (GameSpy)
S.T.A.L.K.E.R.: Clear Sky (GameSpy)

Editions sold in Russian-speaking territories:

Mount & Blade: Warband (StarForce)
Mount & Blade: With Fire & Sword (StarForce)
Eador: Genesis (StarForce)
[/i]

While some people have been able to redeem games outside those restrictions (e.g. Stalker steam keys) there's no guarantee it will work for everyone. You could try emailing Taleworlds and ask for a Gog code, but again, no guarantees.