If you took advantage of the "Join IHG One Rewards Club and Get Diamond Status" deal in Oct 2022, then you get a free upgrade to Hertz Gold Five Star status.
Login to IHG rewards and go to the reward wallet.
You will see the upgrade offer in this section.
Enjoy!
It told me my Hertz membership number wasn't valid, and has now disappeared from my rewards wallet - anyone have same issue?