That question came up in a recent live stream. The staff don't know the answer yet. They are concerned that some people who already use their steam key will simply sell their GOG key to other people. They have no means to enforce that people who use the GOG key are the same people who used it on steam.

I'm also in the camp of wanting my elite founder key redeemable on GOG.com.  The problem Stardock has is with people trying to resell the key to GMG or cdkeys.com among others.  If push comes to shove, I will do the following: buy the base on GOG.com, copy everything else over (thus hopefully including the DLC now and in the future), and see what happens.  I'm in the boat of wanting to play SOASE and it's expansion via GOG galaxy (hopes for non-expert human people and a more populated MP) but I'd need to have the keys for the 2 expansions and the base combined into trinity before I could redeem it.  If key redemption could be restricted to a single IP address that might be while not ideal, better.  However, if they could restrict it to a MAC address, that may be slightly more ideal as it effectively blocks out reselling.  Also, not all founder's who paid $99 gave 30% to Steam-I bought it direct from Stardock, giving them my $99.  We're not asking for deleting the entry just a extra addition-see the S.T.A.L.K.E.R series and the Witcher series on Steam as you can redeem those keys on GOG no problemo.