Looks nasty...
#1 - I'm not an expert in this area...
I would guess that the best way to fix this would be to cut out the damaged sheet metal and tack in a replacement...from a donor car or junk yard. I would consider a cut from just in front of the damage at the fender lip straight up to the rear hatch...hiding the seam under the hatch - around the rear behind tail lamp, then under the body molding back to the fender lip. <just a thought>
Of course, corners could be cut by trying to bang the sheet out from the interior and use a slide hammer to pull the rest. You're likely to need more bondo for this solution and it's going to take an experienced professional to get it right.
My preference is to use as little bondo/filler as possible...which is why I expressed a preference for replacing the sheet metal. In this scenario, I would only need to work the welded seam...and not spend alot of time trying to get the fuel door and it's frame straighteded out.
Both of these methods will require much more time to the body work...it won't be simple, but can be done.
As with anything... you're mileage may vary.
Good luck !
-D