Power systems can and have been impacted by harmonics generated during geomagnetic disturbances (GMD) events. These harmonics are generated from the part-cycle saturation of power transformers caused by the flow of Geomagnetically-Induced Currents (GIC) through grid-connected transformers. This paper describes a system level harmonic analysis tool needed by the industry to perform an adequate assessment of GMD-related distortion impacts. When the results of a properly performed harmonic assessment are combined with the fundamental frequency analyses, a much more accurate evaluation of grid security during GMD can be obtained. The tool is publicly available for engineers, students, and researchers. It provides multiple examples, including a validation against published measurements from an experimental setup consisting of two back-to-back connected transformers in the Fingrid power network. The last section of this paper briefly discusses the results of that validation.