The sticker and metal need to be warm before starting, so if it's cold outside you will need to be in a heated garage and use a hair dryer or carefully use a heat gun to warm the sticker and adhesive.
Pick at a corner with your fingernail until you can lift and peel it off.
Many of the various adhesive removers/solvents will work including Goo-Gone, Oops, and even WD-40 works (it's a better solvent than a lubricant).
Once you get the sticker off, you should wash the tailgate to remove any solvent residue. The solvent will also remove the wax so you will need to put on a good coat of your favorite wax.
...Rich