g-force calculation is basically calculation of acceleration, so you need to do some basic physics math:
1-get the velocity vector from rigidbody
2-tranform into local space to get local velocity
3-find the difference between last frames local velocity (dv)
4-find the acceleration (a = dv / dt)
additional steps to get g-units.
5-convert units to m/s^2 since 1 unreal unit is 1cm, so scale the a with 1/100
6-divide a by g (gval = a / 9.8)
when you get the gval vector you can visualize it via some UI element.