If gratuity is already included in the bill, it generally means that a tip has been automatically added. This is typically done for larger parties (often 6 or more), at establishments with certain policies, or in some countries where it's the norm. The exact amount for this pre-added tip can vary but it's commonly between 15-20% of the bill.
In such cases, you're not obligated to tip additionally since you've already been charged for it. However, if you feel that the service was exceptionally good and want to show your appreciation, you could certainly tip more.
The amount for this "extra tip" is entirely up to you. Some people might add an extra 5-10%, some might add a few dollars, and some might not add anything at all. It's a personal decision, based on your satisfaction with the service and your own tipping customs.
Remember, tipping practices can vary greatly by country, so it's best to familiarize yourself with the local customs when traveling.