I'm not trying to tell GOG how to run its business, I only vote with my wallet.
My understanding is that Valve doesn't get a dime directly from the HumbleBundle Steam key sales, so in that sense probably GOG wouldn't get either, if they offered (free) GOG keys to HB.
The reason for Valve to let developers sell Steam keys free-of-charge (e.g. in HB, or directly from the developer site) is to cement Steam's place as the de-facto platform to play your PC games in. Whether the same tactic worked similarly for GOG (before they'd ran out of money), hard to say.
For many the additional GOG key (on top of the Steam key) might just be worth nothing, they e.g. weren't ready to pay any extra for the GOG key, just like they wouldn't if the games came also with GamersGate keys. It is the same game, just on different stores. At best, the GOG keys would possibly be just sold or traded, like some are now doing for Steam keys.
Alternatively, it might work, and bring more potential customers also to GOG, who would also buy directly from GOG. Hard to say, GOG needs to figure it out themselves, it's their neck on the line, not mine (or any other non-blue writing to this thread). For someone like Valve who has the dominant role in the market, it is far easier to pull stunts like these (stuff for free, or at least heavily discounted, in order to keep the competition at bay).
The free Witcher GOG keys to Steam-version owners of the Witcher games were a very restricted version of that same tactic, ie. trying to lure some Steam users to GOG service as well with free GOG keys. It was easier to arrange, as the publisher of the game and GOG are so closely related, a bit like how Valve would offer free Steam keys for Half-life and Portal games (like they've done before for Portal at least).
Post edited February 04, 2015 by timppu