Xray4277,
You make a valid point, except that boost pressure gauges are not "gauge pressure" devices. They are reading absolute pressure, but 14.7 psi is subtracted from the reading, to give a nominal "zero boost".
The boost pressure on the ground at sea level will not be zero, unless it is a standard pressure day, just as manifold pressure gauges will very rarely read 29.92" of mercury. The gauge on the 182 I fly usually reads about 25" before start.