My Walkera G-2D showed up today. I did as you did and reversed the top bracket, it seems this is what you are supposed to do for install on an FC40. There are no mounting instructions, so it does not say NOT to do this, and the holes are symmetrical, so it lines up correctly and does not interfere with the battery door at all, it's not close to being a problem. In fact, I can't imagine that Walkera intends for you to install it on a Phantom FC40 with the bracket as it comes out of the box, it just is not logical to have it blocking the battery door, or having to use only one screw. As for this humming/vibrating, mine only does that if you power up the FC40 when it's not sitting level. If I have it sitting on a level surface and power it up, the gimbal initializes and the green light comes on. If I lift the Phantom up and move it around, the response from the gimbal is smooth and controlled. IMHO it does not look "hacky" as some have said, it looks very slick actually. I have played with manual adjustment of the pitch and tilt and those work fine also. I need to wire in a connector, as I did my testing with a battery connected to the gimbal. I will test it in flight and see if I get any of the shaking or jello effect. I also went with extended landing legs so clearance is not an issue at all.
I know that people like that one video of the guy with the taped-up camera and one-screw install, but watching it now makes me kind of wince...