Access your War thunder root files on Steam:

*Right Click you War Thunder icon on your Steam Library, then select Manage > Browse Local Files.
My example is in portuguese, but those are the options.

Locate the War Thunder Launcher in the game's root folder

The file name should be "eac_wt_mlauncher.exe".

Setup the War Thunder Launcher on your Desktop.

This will give you a shortcut to oppening the game in your desktop, instead of having to access the game's root folder every single time to open the game with your gaijin account.

*Right click the game's launcher then hit "More Options">"Send to" > "Desktop (Create shortcut)".

Logging in with your original Gaijin Account:

In this step, you will Finally log in with your Gaijin account. When the game Opens up, after you initialized it trough the Launcher, a window will pop up asking for your Gaijin Account and password.

*Fill the required boxes with your data.

*Optionally Mark the "Automatic Login" box so you don't have to write your e-mail and password everytime you open up the game. After that, hit "Log in" and Enjoy the game.

PS: Important note ---> Even after you followed all these steps, if you run war thunder trough the steam library, the game will just log you back into the War Thunder Account associated with your steam account (That annoying lvl 0 Account). So, in order to keep playing with your gaijin account, you'll need to always launch the game via the Launcher (See step 3 for ease of use).

This also means you'll most likely not get any achievements on steam while playing the game trough the launcher, nor will steam account for the hours you've spent playing the game. This is just a turn around for those who have played war thunder via the launcher before it was available on steam and do not wanna loose their account's progress.