When there’s a concert you want to go to and your best friend gets tickets and takes you gratis, be grateful: that means you’re going for free.
You can use gratis as a synonym for "free" or "without payment" as either an adjective or an adverb. For example, you buy a new smart phone and immediately download several free apps. You get your first bill, and see you’ve been charged for all those apps. When you call the phone company, you can say, “Those apps were labeled as gratis. I downloaded them gratis. Why are you charging me?”