They have the same meaning. There is a slight difference. The first sentence uses the future tense (going to), whereas the second sentence is saying there is a plan to leave. We often use present continuous to describe a plan. For example, "I'm coming home next weekend." This says there is a plan to go home in the near future. It's very similar to "I'm gonna come home next weekend."

Gonna is not acceptable in British English. It’s a way of saying « going to ». The second option is perfect.

我们很快就要离开这里和我们很快就要离开这里有什么区别？