Watermelon gelato

Much, much easier to make a sorbet out of melon than gelato. Unfortunately melons tend to have far too much liquid, and even if you try to reduce it (which unfortunately damages the flavors you're trying to preserve), the addition of the reduced watermelon will form unwanted crystals even in a really good ice cream maker.

You basically have two options - make a watermelon syrup as described on this lovely website: http://www.hipgirlshome.com/blog/2011/6/23/preserving-watermelon.html
and swirl it in to your gelato, OR use a watermelon/muskmelon flavored liqueur, and basically use a gelato base for fior di latte or lemon. I think you'll get close to the desired result that way.

these are all great ideas, but if you’re not bent on gelato but would like any frozen treat, try a granita instead! it’s basically a watermelon slushy: the pulp is pureed, mixed with a bit of sugar (but with a perfectly sweet, ripe watermelon you really don’tneed much), and frozen in a shallow dish, while you “grate” the developing ice crystsls every hour or so with a fork until you have that lovely slush consistency.
obviously, this is very different from gelato, but it is super refreshing and incredibly flavourful!

Not sure if you want to go to this extent, but you could put up to two TBS of watermelon liquor to one qt base without risking the texture. I have only done this with a custard base, not gelato, so you might want to research first.

deep freeze watermelon till it's hard like ice cube, remove it from the freeze and throw it immediately in the multi. Break it down to powder and add it to your gelato.