I have been doing paintless dent removal for 13 years, and I will tell you that dry ice does not work. If it did, I would not have a job. I have repaired a couple of gas tanks over the years, and they are very difficult. The best advice I would give you is to look for a PDR guy in your area and see if they can fix it or if they know someone that can.
The dent has to be shallow and not a crease. If it's creased this won't do it.
Remove most of the fuel from the tank. Heat the dent with a hair dryer for a few minutes. Then take a can of canned air, like you use to clean off computers, invert the can so liquid comes out, and spray the dented area.
They say to do it a little different than others. They want it in the bottom of the dent, preferably from the back side which makes a bit more sense. The cold will shrink the metal which has been stretched from the dent. How well it works would likely depend greatly on the dent and the metal thickness , etc. There is no blanket statement that applies to every dent. It may or may not work, depending on your dent. I think it may be worth a try.
DENTS & HAIL DAMAGE Dry Ice will condense metal and thereby shrink small dents on your car. Place the Dry Ice on the inside of the dent if possible. Use heavy gloves and press flat sheet against dent. If it is not possible to get on the inside concave part of the dent, then using heavy gloves hold the Dry Ice so a corner can fit into the bottom lowest part of the cratered dent. Hold the Dry Ice until the metal is frosted at least 2 inches beyond the dent. Let the metal warm up (in the sun is the best) and repeat the procedure. Sometimes the dent will pop out perfectly. More often it will not be possible to get a flat smooth finish, but the dent will be reduced noticeably. Creased metal will still show the crease line but the dent will be far less pronounced. I have not seen any paint damage, but I'm sure if the paint is not strongly adhered, it could peel away.