This account is already linked issue. Even emailed Niantic support bit they can’t even find a solution too.

So everytime I try using my gmail to link the account to the gmail, it shows this.

Even Niantic support confirms that that Gmail is not linked with any of the pokemon go accounts.

And they just say I should try to link the account with another gmail. (I don't want to do that as I want to link the game to this account only.)