what gauge steel I should be using.
its not hyper critical, but heres a few things to consider on that topic.
if its undercoated it will be rotted back away from the edge. cut back to the fresh stuff or to the nearest fold.
all panels will have something like ribs, hats, or steps formed in them to give them rigidity. you dont have to make an exact copy of whats missing, but the bigger the hole, the more this becomes a consideration.
usually hole patches provide a great new place for moisture to hang out, (just like in all the factory spot welded locations). if its a long term fix, you might want to think about using a bodyshop grade panel epoxy or using seam sealer then rivets to squeeze out the excess.
then again, theres always duck tape if youre in a hurry