Excellent idea. And it works pretty well. I think I'd like it better if the gate palette were visible the whole time while you're in building mode (not just when the mouse moves up). Maybe it could have a different background to show that it's not part of the circuit. But it would be much clearer what to do.

Fixed a bug: when gold marble drops, don't lose the board immediately, but wait until other balls stop to see whether the board is cleared.

I think you should turn the default speed down to somewhere close to the middle. This runs way too fast on my machine without adjusting this.