Adding a Gratuity when you have enabled Pre-Authorized Card Payments with Authorize.Net is very simple.
When Authorization of a Payment is obtained from the Customer, the amount which is Authorized is always 120% of the Order Value.
Thus, if the Customer adds a Gratuity at a later date, they can do so without having to provide fresh Authorization.
Remember, however, that you must Charge the Card within 30 days of the Customer providing Authorization of the payment. This is because Authorization of a Payment can only remain in place for 30 days.