When we speak about future arrengements)
OK, we use <em>to be going to [verb]</em> when we have plans. The main point here is that it is possible to change your plans and nothing is "set in stone", so to speak.
When we use <em>to be [verb]ing + future time</em>, this is an arrangement. This means that a set time and place have been decided, and maybe other people are involved in your plans and they know about it. Because it's a commitment, it is not possible to change your plans (not easily, at least).