"What's the difference between 'I'm gonna miss you' and 'I miss you'?" that's the correct spelling for the question :) and when you say 'I miss you' you are using simple present, it means you are missing a person in this moment. When you use I'm going to miss you (contraction I'm gonna miss you) you are using future progresive, it means you will miss the person by the time, it is when you use verb to be, verb with ing and using go (going) means a future action... I hope this helps!.

'gonna' is a contraction of 'going to ' which refers to a future action. I am going to miss you ( when you leave I will miss you).So the person didn't feel it yet. The friend or lover is still there and didn't leave yet ! I miss you is a present feeling at the moment , the speaker feels it because his friend /lover ( the missed one) is already away and therefore being missed. Or He just met a friend or lover he didn't see since quite a long time and expresses at the moment the fact that he missed the other all the time he was away.

I think walter's answer is good and right