When running leftwards, the shadow is to the right, the same as running rightwards.
As a patch I tried making new files with mirrors of *_running_shadow.png and *_running_gun_shadow.png and referencing them in the flipped_shadow bits of the code.
I'm not sure if I should submit this to the GirlCharacter mod too - it seems inactive, but SleepyEngi would have the 3D models if anyone does.