Hi, thanks for your reply.
I am the Gold member, and I am underage
I've been told on the phone (by Italian Hertz) that the coupon is only for US residents.
And I was told by live chat support that it's only for family members...
My reservation currently has the CDP 1911795 which also waives the underage fee. But I read that some people were denied to use it because they didn't have corporate ID. See
http://www.flyertalk.com/forum/16471003-post99.html