There is a difference between the present continuous and the 'be going to + verb' structure. The first shows an action that is taking place right now, but the second shows intention to do something in the future. Therefore,
We are going home = Πηγαίνουμε σπίτι
We are going to go home. = Πρόκειται να πάμε στο σπίτι.
Plenty of examples of sentences like "We are going home" to refer to the future.
This whole skill seems a bit odd - I've rarely heard πρόκειται να in real-life Greek, I've much more heard the future tense used to describe something that someone is going to do. Is there a particular context that πρόκειται να tends to be used in?