Please note, if your Gaijin account was created via Steam or console (PS/Xbox) version of the game, before linking War Thunder Mobile account you would need to bind an email address as a login for console/Steam War Thunder account, as instructed in these articles:
If, during the first sign-in, you created an account via Google/Apple/Facebook login, you can link your game account to an existing Gaijin account.
1. You can only link a Gaijin account that was never used in War Thunder Mobile
2. You cannot link a Gaijin account if you already linked an email as a login to this game account.
3. At the moment it is impossible to access the game using linked Gaijin account login.
4. You can link only one game account to the same Gaijin account.
If you agree with these conditions, then you can link your game account to a Gaijin account via account's section in the game, by pressing "Link email" option, and writing the email of Gaijin account: