"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!.
