I'm gonna do it.

The phrase "I'm gonna do it" usually refers to a specimen of the human species verbally expressing the phrase in reference to do something a friend or partner has told them to do. Most of the time, the friend or partner is joking because the thing being referenced to is risky or dangerous to the person or people around them.

Person 1: I dare you to jump off that building.
Person 2: Sure.
Person 1: No wait, don't actually do it.
Person 2: I'm gonna do it.