Linking your Gaijin account to Steam is necessary to synchronize the progress of Gaijin and Steam accounts and is possible provided that you have never entered the game through the Steam authorization button.
If you launch the game through Steam, and have never logged into any account of the game through the Steam client before, then to link the accounts you just need to specify the username and password for your Gaijin account and click the "Login" button.
After that, your account will be linked to Steam and you'll be able to enter it through the Steam authorization button, as well as receive purchases on it from the Steam store.
If you have already logged in via Steam client of the game or accidentally created a new account that you do not need through the Steam authorization button, then you need to contact our support service, as described in this article. Please describe the situation in detail in your request and attach a link to your Steam profile.