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I would like to activate this product on steam but I cannot find the product code that it requires. Maybe I am a fool and GOG does not give that option, but hopefully I am not completely.
If you want Steam key, then you should have bought the game from Steam. GOG is independent service, which doesn't have anything to do with Steam, so they don't provide Steam keys. Quick browsing tells me that most other online stores won't provide Steam keys either, because this isn't Steam only game. However you can still play the games via Steam launcher if you use launcher's "Add a Non-Steam Game to My Library" -function.
GOG has a pretty strong NO DRM statement, why should they provide steam keys? ;)
Post edited January 17, 2014 by Sirkenneloth
Dude .... Your at the wrong company if you want steam keys !
DRM is that a way ---->
"Quick browsing tells me that most other online stores won't provide Steam keys either"

From my experience, they do. At least the sites I use do (those being HumbleBundle and Greenmangaming). It's not unheard of for websites to provide both a DRM free version and a Steam Key.
In fact, for a lot of people the Steam Key is for sheer convince of having everything organised with automatic updates, in some cases the Steam Workshop access, easier access to your game library on different computers (assuming they already have Steam installed) and access to a few other features only available via Steam such as achievements and trading cards (two things personally don't care about) whilst the DRM-free copy is for piece of mind that no matter what you have a copy that is 100% yours.
Steams DRM isn't exactly restrictive. It's DRM no matter how you look at it but it's actually a pretty consumer friendly version of it since I can't think of a time it's ever stopped me gaming.

As for the answer to the original question:
(though it's been a while and I expect no response from him, this could be helpful for other people).
Contact the developer via email. An email address should be available via the Devs own website. Just ask them if there is a possibility of getting a Steam Key and provide proof of purchase (the email you received when you bought the game). Most developers are more than happy to hand you one provided you can prove you bought the game AND the game is actually available on Steam.

I personally like to have a Steam version of things for ease of use and convenience when available.
My point here being, I've contacted a few developers asking whether a Steam Key would be possible and I've been given them.
It depends on the developer as a few of them have said it's not possible, even with my proof of purchase. I'm not going to say "it depends if the developers are nice" because I don't know what happens with keys and what it takes for them to give them to me and I'm not even going to pretend that I know. Be thankful when/if the Devs actually give you one.

Moral of the story being, if you really REALLY want a Steam Key, make sure the website you're on gives them to you in the first place. If you see a deal on here that's better than other places be sure you're not bothered about a key since getting one from the devs isn't a 100% guarantee. Back to the Future isn't much of a problem IMO since it has NO Steam features.

PS: From personally having a go at getting a key for 'Back to the Future' from the devs, I can tell you now that you won't get one.
Post edited June 16, 2014 by linkno1