wonbon: Well, I'm just bringing an active problem to light. And like so many other users of forums, I'm just looking for a solution. As it had happened to other members as well. Sorry if I offended you, but no need to get passive aggressive about it.
Two potential solutions (you can even use both if you want to):
1. Use (change) your GOG password to an unique one you are sure you aren't and haven't used anywhere else (on any other web site, forum or service), and make that password strong too (=long).
2. Enable the optional two-step verification in the GOG account settings.
As for a solution to your already hijacked account, you will just have to wait until GOG support responds and/or reacts. Of course they somehow have to make sure first that you really are the legit owner of the account, and not someone who tries to hijack it (with GOG's support). EDIT: So they did already.