If you want to call it a gratuity, then let the customer decide...afterall the meaning of the word gratuity is: -
1. a gift of money, over and above payment due for service, as to a waiter or bellhop; tip.
2. something given without claim or demand.
But if you have staff to pay or fuel and you currently don't factor that additional cost into your running costs, and so calculate it as a percentage above and beyond the bill, call it a "service charge" and charge it anyway.
But I'm with Big Mike and Bill the Grill, if you call it a gratuity, I will decide if I pay it, if you call it a service charge, and you keep it inside the industry standards.
I have always called it a gratuity, and left it down to the individual, but then I price everything and factor that into the costs of the bill. I don't do the gig to depend on the tip...if you have to depend on the tip to make a profit, then your prices aren't high enough! IMO.