Protoss: They could make a deal where you send a proof of purchase on GOG to Valve or something. It would be possible with credit card, if you ever bought from Steam. ^^
And by the way, you get the preorder bonus without actually buying it from GOG later.
They could, but nobody knows now. Maybe even they don't have decided this now. Just wait until the game is out.
Here is the plan: pre-ordering on GOG doesn't mean you're obligated to buy the game. You can decide later. I know this for sure because I pre-ordered accidentally (shame on me) and panicked and asked them and they told me so. I actually even might make the deal, if first user reviews are really good.
So, just pre-order now from GOG. Then after 17th of May when the game is released and Steam achievements work, ask them if you get an additional Steam key. If yes, buy it here. If no, decide what is more important to you.