I haven't found a solution to this either.
obliviondoll, thank you for reply. I don't know how they (Gog team) do this. I have already look over all folders of Galaxy, that I know, but I don't saw nowhere such background (blue color) and text :(. So, unfortunately, don't know how to disable this "banner".
My assumption is that the client is getting the banner from GOGs website and it is not coming from inside the client itself. I verified this theory (at least partially) this by installing older clients (1.2.64, 1.2.66) and I still got the blue banner displayed.
So i assume that GOG, at some point after the release of Galaxy 2, decided to display the banner in all 1.2 clients, maybe depending on the user-agent, the Galaxy-Client provides with his request? This would mean that GOG could disable this "feature", if they wanted to, which they obviously don't.
Looking at the lack of reaction to anything that happens in their own boards, i strongly doubt that the banner will be removed ever.