Getting back to trying to propose practical mitigation strategies for goodhart's law, I propose a fairly simple solution: Choose a G*, evaluate performance based on it, but KEEP IT SECRET. This of course wouldn't really work for national scale, GDP-esque kind of situations, but for corporate management situations it seems like it could work well enough. If only upper management knows what G* is, it becomes impossible to optimize for it, and everyone has to just keep working under the assumption they're being evaluated on G.

Taking it a step further, to hedge against employees eventually figuring out G* and surreptitiously optimizing for it, you could have a bounty on guessing G* - the first employee who figures out what the mystery metric G* really is gets a prize, and as soon as it's claimed, you switch to using G**