Modify GUI.color before the button call, then set it back to Color.white after (or else all the rest of the gui will do it too)

GUI.color = new Color(1,1,1,0.5); //0.5f in c#
GUI.Button(new Rect(50, 50, 200, 50), "Semi Transparent");
GUI.color = Color.white;

GUI.color.a = someFloatValueBetween_0_and_1

This will directly control the alpha of anything after this line.

Be sure to set the GUI.color.a back to 1 so the remaining GUI Elements aren't dimmed as well.

i need to make totally solid button