Ok... so this is on my pre-order then.
I was lucky enough to find time at work to watch the Live Stream from Games Workshop about this book.
Their quick highlights were:
3 new Minatures will be released with it (can anyone say Primarch here?)
It will also contain rules for playing games around the Cadia Gate
Finally it will contain rules for ALL 40k armies to be used at the Cadia Gate.
Yes I can see myself getting a pre-order for this and the other Gathering Storm books in this series.