What this doesn't get across is that the bones on each rig are oriented differently to each other, and the amount of difference varies wildly.
Like, one GE bone might require each axis to be cycled. So GE's X axis matches HL's Y axis, GE's Y matches HL's Z, etc.
Another GE bone might match the X axis on a HL bone, but Y and Z will be swapped, and Z will need its values inverted.
Basically rules are fake and none of this is worth the end goal (Goldeneye anims in HL), but I'm GONNA DO IT ANYWAY