"Half again as much territory has been brought into subjection since Gaugamela as in all the prior campaigns."
That convoluted passive-voice sentence is indeed extremely tough to grasp. It might help to reconstruct it as follows: "Since the subjection of Gaugamela, again as much as half of Gauagemela's territory has been brought into subjection in addition to the territories brought into subjection by all previous campaigns." Even in this form, we really can't be sure if we have gotten the sense of the original statement correctly. A major problem is that it isn't clear who is doer of the subjection; it's also not clear if territories other than Gaugamela have been involved in the subjection.
The best thing you can do is to ask the author and publisher of that book to clarify that statement and its nebulous mathematics for the benefit of its readers. It would be great if you can share their response and revision with Forum readers.
