I’ve just changed the device I play War Thunder on, (I previously used my own Gaijin account through someone else’s Steam account, now I have my own Steam account) and I can’t log in. There is no “Switch User” option for me in my profile, and the auto-login option is turned off but I’m never brought to the login page, just automatically signed in on a new account the game has made.
to get to your Gaijin account you may have to run the game through the WT launcher (‘update the game manually’ ón Steam (you may have to click on the small arrow next to the play button in the library) or just run the launcher.exe in the WT main game folder)
when the launcher starts it may try to update the game to the ultra high quality version (+65gb). in this case you have to change the client to ‘full client’ in the lower right corner of the launcher (unless you have the (free) UHQ DLCs installed on Steam)
once the launcher has done its job and the game starts you can enter your Gaijin login credentials on the login screen.
unfortunately since your accounts aren’t linked you will be logged into your new / fresh Steam account every time you start the game directly through Steam (without running through the launcher) i’m afraid… if this is the case and you want the Steam account linked to your Gaijin you will have to contact the customer support - https://support.gaijin.net/hc/en-us/articles/360000262858-How-to-submit-a-support-request