Leroux: Supporting a company out of your free will (like you do in your example) isn't, but the company making it a condition for you to get something good in return is a bit different. You may not even like the company but "support" it only because they promised you something shiny. I don't know if that's immoral, it's regular business, but I still consider it kind of shady myself, or in any case not sympathethical. And isn't this a bit like offering money to your loving partner and ask them to give you a good time? ;)
I think you are over thinking the issue. Try it from another point of view. If you like GOG and would support them on your own free will, here is an opportunity they have for you and they would also like to make it worth your while.
What I find to be rather silly are the companies that sell their logos on t-shirts, mugs, etc. YOU pay THEM to advertise for THEM. :/ This is an attempt to flip that crazy setup in favor of the fans. If you want to advertise for us... let us do something for you.
The fact that its optional should make this discussion moot. I don't think there is anything immoral, underhanded, or remotely dishonest with letting people know what they can do for you and what you are willing to do in return.
Bottom line, if you don't feel like telling your friends about GOG, you should not utilize the social aspects of this contest. If you do want to tell your friends about GOG... bonus!