The Gantry Pitch angle is not defined in IEC 61217. This angle is defined in the DICOM standard in a way compatible with the current notion of IEC by introducing it as rotation of the IEC GANTRY System as indicated below.
The Gantry Pitch Angle is defined as the rotation of the coordinate axes Yg, Zg about axis Xg by an angle (g; see Figure C.8.8.25-7. An increase in the value of angle (g corresponds to the clockwise rotation as viewed from the isocenter along the positive Xg axis
Figure C.8.8.25-7 Gantry Pitch Angle