I should probably field this post by Eric B, but Orangey has the opportunity to respond to dissonance if she chooses. And I agree with jason m's post, for the most part.
To define I/E with something as specific as "gregariousness" is erring, I believe. I can usually tell the difference between an I and an E by watching them communicate IRL, but I can't give you a short list of specific traits. How I would best define I/E is need, and it can sometimes be noticed. Viewing an individual in group interaction, say, do they seem to need the interaction? If so, E, and if not, I.As for I/E; I would say based on the type descriptions that your "expressiveness" will basically match the dominant function attitude, though for various reasons, it may not always play out in the stereotypical "introvert/extrovert". So an extravert may be more reserved at times, but the "gregariousness" will come out in some way at times, or they will be driven by the same motives, but carry them out differently. (Much of this questioning of I/E definitions involves saying "I know this Exxx who is quiet and thinks a lot"; but this is basically the exception and not the rule, and probably also you're only seeing a period of their behavior. But their dominant function is defined as referencing the outer world of people and action. (and there's also the possibility that they're simply mistyped).