You don't mean fender skirts, you mean wheel wells, don't you?
A fender skirt is a filler for the wheel opening in a fender. They were a popular accessory and factory part for many decades.
The wheel wells are the portion of the body behind the rear fenders that the wheel assembly moves up and down in when the car is moving. These are often rusted away due to years of not cleaning the mud/dirt/road salt from under the car.
16 gauge steel should do you just fine to fabricate repair panles for your wheel wells.