"gonna" is slangy

Hello Nony, in meaning, there is no difference since "gonna" is simply a very casual way of saying "going to". But there is a big difference in usage. "Gonna" is much more casual and used often in speech, but not commonly in writing. "Going to" is more formal and used in both speech and writing. Hope that helps.

Never write gonna unless you put it in quotes "gonna" and clearly indicate you are saying how somebody else spoke. Spoke = past of to speak.

I believe it's American informal saying of be going to