I've been planning an RTW *A trip as well. The *A RTW planner provides miles, which I hope are accurate. I've not heard of the GCM tool, but in order to get exact mileage, I crack open Google Earth, and use the path tool to chart point to point (great circle is always used in measurements).

Moreover, I zoom in and select the middle of the longest looking runway as my start/end point. If not exact, it's got to be pretty close in mileage.

Eric