When you had a job for the monthly income from some companies over a period of 2 years. Then the account was enroll as the salary account. So then if you withdraw the money from casino it was seems to be the funds from your salary. Even it may not from the income of salary, this case make to pass such things due to the old transaction. Then it became easy to withdraw the gambling money. No need to pay separate tax for this. Its best way to use the salary account for the gambling site and to win. When the pass of bank account was approved, its essential one for the gambling withdraw money from it.
I think what casinos are looking for from proof of income is to check whether the player pays the right tax or not from their winning from the casino am sure casinos only ask for such documents when the player already has a history of past big wins and withdrawals.