I attempted a gradient a while back with blues, that worked okay and looked good from about a foot away. For that one, I painted my nails a cobalt blue, then sponged a medium blue over 2/3 of the nail, and then sponged a light blue-gray over the tips. This was the result:
Fire colors are my favorite, so I decided a fiery gradient would perfect, esp with the heat we've been having!
Using Essie Fear or Desire, a lovely bright orange that isn't quite neon, and Sinful Colors Ruby Ruby for the red, I got this beautiful gradient!