If you use the frequent traveller retro credit on the website it credits within a few hours (perhaps overnight depends on its mood) and without any problem. All you need then is the invoice number, date and your own details. Its very easy, you may wish to give it a go - I appreciate that this may not work for Gaum (I've never tried) however it does not make any exclusions on the website itself and I've had no problems using it for some more exotic countries in the world.

You can access it here:
https://www.hertz.com/rentacar/partn...roFormView.jsp