The biggest thing about any kind of petroleum product spill is the smell.
You have to remove anything that the gas had contact with, all carpeting, all plastic, spare tire, jack, etc.
The only thing left should be the unibody of the trunk itself.
DO NOT ever vacuum gasoline! Mop up as much as you can with rags and or paper towels and dispose of them properly.
Use a good quality degreaser on the entire trunk. If you can find the drain plug for rain water, most trunk's have some sort of drain, remove it. Now you can either pressure wash the trunk, or just flush out any remaining gas with a regular hose.
As far as the other items, especially the cloth, or fabric components, the best way to remove the gas is again with a good degreaser and a blast of hot
soap from a SS car wash in the bay. You may have to do this many more times than just one time. Once you are satisfied that the gas is gone, wait until the item is totally dry and sniff it for any remaining fumes. If you still smell the gas repeat the whole process. If you don't get the smell out completely, and proceed to put the trunk mat / gas soaked items back in the trunk, the gas fumes will never go away.
We have done this many times at our detail shop and have found this to be the only sure fire way to guarantee the results.