The question is; do you care about having a drain in the gas tank. If you do, then the best advice is to get a sheet metal bolt that can thread itself into the hole. You will want one that is big enough that it's going to be a TIGHT fit. Also, one with a big head on it so you can get a neoprene gasket to put over the bolt before threading it back in.
If you don't care about a drain plug; or at least want a temp fix until sometime later when you can drop the tank, then fiberglass is your friend. I launch one of my cars over a curb once when a tire blew out. It dented in and punctured a hole about the size of a golf ball in the bottom of the gas tank. I got a fiberglass repair kit; (Not the bondo type putty). The kind that has the resin and sheets of fiberglass cloth. I mixed up some of the resin and I took a couple of sheets of the fiberglass cloth, and I shred it up. (It's all threads and pulls apart into fibers easily). I then mixed the fiberglass threads in the resin and made a "GLOB". I used this glob as a PLUG. About 1/3 actually in the hole and the rest on the outside. After it somewhat hardened; an hour or so; I then mixed some more resin and actually applied the fiberglass sheets of the Hole/Glob/Repair job. This filled in all the other areas and made a nice smooth transition. It was suppose to be a temp fix; but it stayed that was for 6 years until I sold the car.
You can get the fiberglass repair kit at Wal-Mart or probably any auto store. It's great with gasoline. (Never saw a corvette melt spilling gas on it when filling up; have you?). It's cheap and it's very permanent if you want it to be.
If you want a semi fix, skip the "Glob" portion and just fiber glass some sheets over the hole. Then later, it will be very easy to still see where the whole USE TO BE and you will be able to drill it out. Personally, unless you really have need of draining the tank a lot, just plug it with fiberglass and seal it. You can always siphon out of the tank if you have to drain in. | 
