Originally Posted by AutoSlash

Unfortunately, if your Hertz Gold# is not attached to the reservation this typically is not possible. The gate agent will probably turn you around and send you to the counter, although there isn't much harm in trying other than pissing off the people behind you in the exit line.

Btw, National works like this. Regardless of how/where you booked, if you are an Emerald Club member, you can just grab a car and drive to the booth and they can process everything there. They will even associate a prepaid opaque or whatever reservation with your EC# and you'll get credit, etc. Hertz, unfortunately, does not work this way. If it's prepaid and your Gold# is not attached at the time of booking (possible in some cases when booking through Priceline non-oqaque) then there's no way to do it afterwards, even at the counter.