Hi,
I know you provide Steam keys for purchases through most other advenues but do you provide Steam keys for GOG copies of games? I believe Steam do allow you to provide keys at no cost (unlike GOG who often only provide them in certain cases e.g. purchases before the game was available on GOG) but I understand there are reasons you may not want to do so. In particular, while I can provide proof of purchase from GOG, I'm not sure how easy for you to check.
My Steam profile is public so you can also verify that I'm adding it to mine and not selling it. (Well I guess you can't verify that I'm not doing something else dodgy like adding a Humble Bundle copy and reselling my key.)
In case you're wondering I purchased from GOG because of the sale and also because I prefer DRM free but I admit I also like the Steam achievements and it's nice to just have it on Steam. If you can't provide keys that's fine just thought I'd check. (The specific game is Primordia although I have some friends who purchased other games. Not sure if they'd want to add theirs to their Steam though as they don't use it so much but thought I'd ask more generally.)