A more thorough explanation:

Definition: Gratis (grat-is or gray-tis) is an adjective that means free of charge or without compensation.

• The company is offering a gratis trial of their new software.
• The concert tickets were given out gratis to the first 100 people in line.

In the first example, the company is offering a free trial of their software without charging any money. In the second example, the concert tickets were given out for free to the first 100 people in line, without any compensation required.