Having Aluminum re-powdered really isn't a big deal. How much would it cost to have it painted? Is it a fancy paint scheme? You might be surprised at the colors, effects and textures available with powder coat. You might find a few phone calls worth your time. There's no less than half a dozen shops in most metropolitan areas. If you're willing to potentially compromise on quality while saving a few bucks, fledgling powder coaters can be found on the local Craigslist.
Another option is to rough-it-up with a Scotch Brite pad, then prime and paint it with rattle cans from Home Depot. If that turns out to be a bust, it can still be re-powdered. It seems the main thing, finding a suitably constructed/configured tailgate, is the difficult part.
If you truly want it painted, a reputable paint shop should have access to metal strippers. Have it stripped and paint as usual. It's only Aluminum, not a kidney.
