I've no idea whether the Kenko contacts are gold plated, but in a side-by-side comparison of the Kenko and Canon 12mm extension tubes, they appear to be identical. One would expect the contacts to be so, but the construction screws are located identically and design of the outside of the mount ring are also identical. Somewhere hey are not exactly the same, the Canon tube is, perhaps, a gram heavier.
I've had no problems with the Kenko tubes which I've had for over two years.
Alan