Well, I supposed you'd answer like this, actually

.
But for me it is difficult to decide from the context - e.g. We're
going to/will spend summer holiday in France. Our friend (who is
going to/will have a baby)
will go/is going to go/ is going with us.
It's very confusing for me... I know all the grammar rules, but I can't use them... It's not that easy for me.
I'll try to guess... I'd put there
going to, will, will go... Is that right? I suppose it isn't...