Question

Only a few AFAIK.

These titles can be redeemable on Steam.

Some titles in GamersGate and GMG will usually warn you if they're Steam-only on their description. If they're not on that list they're more than likely not work on Steam.

Actually, these two posts:

GamersGate games that can be activated on Steam

GMG games that can be activated on Steam

Are better sources. The official Steam article lists games whose keys have to match specific formats - some of those games on GG/GMG have different formats that won't activate on Steam (e.g., Mount & Blade keys from GMG don't work in Steam - source: experience).