It might be that the drill file is calling out the dimensions... I'm not sure, haven't tried to read the raw Gerber text before. I have similar issues when importing some old designs into ADS Layout. By default the Layout editor uses mils, but the drawing is scaled in inches, so the resulting imported polygons are 1000 times too small. To correct this, I simply select all, Edit, Scale (somewhere in there), and tell it 1000x in X and Y, and viola! You could do the same thing here... since it's mm, and you want inches, the scale factor should be 25.4 or 1/25.4, since 1 inch = 25.4 mm. Measure a distance between a few known vias in PADS, and compare it to the length in ADS layout. You should be able to determine the scale factor pretty easily.