How to make a GUI Texture (image) transparent?

Hello! I have a simple image made in GIMP that is gradient, green to white. I loaded this image into my code through the editor, and I need to edit the entire images' transparency.

My code to load it in is simple:

 public Texture GPgreen;

I've assigned my image to that slot in the editor, and need to affect the whole things transparency. It is drawn as follows:

 GUI.DrawTexture(new Rect(commonLeft + padding, commonTop + padding, healthBarLength
                                  ,healthBarHeight), tex1);

Any help is greatly appreciated. Thank you!