Ah well, Megan (and India Four Two) we're just going to have to agree to disagree on this one then!
If I attached a pressure measurement device to any system which was at atmospheric pressure, and the device read zero, any engineer who I asked would say it was measuring gauge pressure, not absolute. If you are subtracting 14.7 psi from the boost gauge reading so that the device shows zero, to me and any other engineer who I know, that's not absolute pressure. The subtraction makes it 'gauge pressure' by definition.