To change the orientation or position of the gizmo mapping, click the 3D button at the top of the gizmo mapping screen. You should a see drop-down list:
Move
Rotate
Scale
Move will move the gizmo, dimensions are in meters
Rotate will rotate the gizmo, dimensions are in degrees
Scale will change the size of the gizmo
To rotate the gizmo properly, see how the green arrow lines up with the texture, then line up the green arrow on the model so the texture orientation is correct. This may take a lot of trial and error. I've done some gizmo mapping before, but I am nowhere near good at it.
You can also change the green arrow's orientation by clicking the SwapUV, MirrorU, and MirrorV buttons near the bottom of the gizmo screen.
Finally, if you need to, you can drag the gizmo screen around by clicking and holding the top blue bar of the screen while you move the mouse. This will allow you to see one of the model views to help get the gizmo just right.
Good luck, what you are trying to do is one of the hardest (IMO) things in O2. Feel free to ask any questions, whether they seem stupid or not.
-Pilot