"When are you going to move?"
"When are you moving?"
the difference between the two sentences is very slight
1 -more emphasis on the intention, the speaker had the intention to move before the time of speaking
2-generally ,it expresses personal arrangement,it is quite informal
"It's going to rain soon"
we use to be going to here to make a prediction based on present evidence
like : the sky is very black
we use it to predict future,especially when we can see soemthing that is about to happen
"It's raining soon"
Perhaps we are not sur,this form is not stronger than the first
I am a non native speaker student, so,I could make some mistakes but I hope that you find it usful,
you can support your comprehension with these books:
Longman English grammar practice
for intermediate studentsby L.G.Alexander
Grammar practice by Elaine walker, Steve Elsworth
They are very useful and very helpful
Best wishes