Both are correct and they are interchangeable. There's no clear scenario where you would use one instead of the other. "Gonna," however, is slang for "going to" and makes option B read more informal.
6. August 2018
A is referring to the future: Me: "One day the Uffizi gallery will sell all of Michelangelo's work to the Americans." You: "That will never happen."
B is referring to the present: Me to woman on Date: Hey give me a goodnight kiss! Woman: That is never going to happen.