Sometimes I hear people who say "we're gonna be talking about" and I really don't understand how they are different from those who say "we're gonna talk about"?" Is it not the same and it's just a matter of style, or there's a difference in fact?
A few decades ago, "gonna" wasn't considered a "real" word. It was used to imitate a sloppy speech pattern in writing. It's been texted into existence and now shows up in dictionaries as a legitimate, but "informal" contraction of "going to". Weather Vane's comment is about right; if you're using "gonna" in a sentence, don't expect the rest of the sentence to reflect the rules of "the king's English".
That said, either version of the sentence in your question would be common usage, the same as it would be using "going to" instead of "gonna".
"we're going to talk about..." is a little more direct, with more of an "action" nuance. "we're going to be talking about..." is a little more of a process nuance. In most cases, either would work, but there are some scenarios where one would be more likely than the other. Where the talking will be two-way communication, the process-oriented version is more likely. When the talking is going to be mostly in one direction, the action-oriented version is more likely.
For example, a parent admonishing a child about some misdeed would probably say, "We're gonna talk about this!", and the "talk" is likely to be mostly a one-way communication. A presentation is likely to be introduced as, "we're going to talk about XYZ." (Nobody would introduce a formal presentation using "gonna".)
A moderator of a discussion group might introduce the subject with, "we're going to be talking about XYZ." The talking will be more of a process.
But in everyday speech, either could be used in most cases.