Hi. I found out about the gog reclaim game thing. But apparently i cannot redeem my steam key of mount and blade warband, with fire and sword, and Classic Mount and blade. Can Anyone help me with this

Only the (boxed copy) Russian version of M&B can be redeemed on GOG.

I'm not sure about all of them, but I'm rather confident the first M&B game was redeemable on Gamersgate. It's been a long while, but I think that's true.

The GOG reclaim game thing only works for boxed copies of certain games, why would GOG let you redeem a key you bought from a vendor they (try to) compete with?

Because it says: "Reclaim your games. DRM-Free!" ??
Because there are people out there who stopped buying from steam, but once bought some games, like m&m wb on steam? Because they won't buy DLC on gog if they can't redeem the already purchased game?

There are some games out there, like AOW3 or m&m wb just to name two examples, which I'v once bought on steam. I do not buy anything on steam, I want my games DRM free. I will not buy the games here for the named prices again, so I also will not buy DLC here.

I think gog is smart enough to see actually loose money this way. Might even be they have that idea in focus but some legal constraints etc. might prevent them from implementing it constistant.

A good first step could be something like that: Buy DLC on gog and redeemthe main game from e. G. steam.