ok firstly remove everything from the rear.
then weld a piece of plate metal the full width and depth of the indentation. (remember to disconnect battery and alternator though
)
then once thats done etc. grind the welds back so they dont stand out
then fibreglass (bridger most of it to get it somewere near. then g7 it down or glass sand it. it is tough to sand.
then once u have it to somewere near u can use a fine cataloy and spread it on then with a square block wrapped in wet and dry sading paper. start to sand get it somewere very near.
then best thing to do is to purchase polyester (you will require compressor to spray this on.
this is a filler but also stops the pait sinking into the cataloy and can be easily sanded smooth
once thats done, you are then ready to prime and spray the tailgate to the aquired colour. remembering to mask of the windows etc
good luck it will take more than a few days to do though.
depending on your timescales as in how long each day your working and how often. u may be loooking at a full week just to do it
wayne