This is simply the verb "to go", ie "to go". It is a very versatile verb and can be used in many situations.
There are several phrasal verbs that use go, but as a verb itself it can be used in situations like this phrase here:
I'll go to the mall tomorrow.
In fact, gonna doesn't need so much explanation, because it's nothing more than going to in a compressed form.
I'll use the examples above to show you how you would use gonna, ok? Come on:
What color are you gonna paint your room?
I´m gonna paint it blue.
What are you gonna do after work tonight?
I'm gonna stop by at my mom's house to pick up some delicious food.
I'm gonna tell dad you lost the RC.