If the Gauls have taken and looted a city it becomes independent.
On a subsequent turn or in the surrender phase can they siege it again and turn it into a Gallic city?
Does a looted independent city immediately gain a siege 0 marker?
My reading of the rules suggests that this is possible, but it then seems that the Gauls can get double value by sieging the same city twice, once to plunder it, and then to hold it.
That is correct, but rarely happens as the Gauls don't get any advantage from holding cities and Gallic cities are capped at loyalty one so you can't make it harder for the owner to take them back. Bottom line is you can do this, but 99% of the time you will have better uses for your actions.