Most reuse the existing gas tank strap. The bolts holding the skid/support to the frame are also generally utilized by the aftermarket part.
Coincidentally, I was in the process of adding an aftermarket gas tank skid (Kilby) to my Jeep just yesterday and broke one of the bolts on the gas tank strap, so now Im waiting another week for the replacement to be delivered. If I had been a bit more patient, I probably could have prevented the extra trouble with some PBBlaster/Liquid Wrench before trying to turn the 10year old rusty nut off the bolt.
Oh well.....its a Jeep....Ive learned to not so much worry when stuff doesnt go as planned, but to worry when something seems too easy.