It seems that you're using an outdated browser. Some things may not work as they should (or don't work at all).
We suggest you upgrade newer and better browser like: Chrome, Firefox, Internet Explorer or Opera

So, I connected Steam with GOG and have all my games visible in the launcher. Now is a sale in the shop and I realized that I can buy games, I already own at Steam. There is nothing that says, that I already own the game. So I still have to switch between the launcher to compare what game I have, and which I don't. I hope you will make something that say: "You already owns it on Platform X"