Navigating helps you get where you're going. When Ms. Cocktail Den and I travel on vacation, she's frequently the navigator. If it wasn't for her, we might still be lost on picturesque desert highways in New Mexico, empty rural roads in Ireland, or the congested urban maze of Bangkok.  The Navigator comes from London, where Jamie Terrell created it in 2005.

2 ounces gin
.75 ounces lupo limoncello
Juice from 1/4 grapefruit

Combine in a shaker with ice, shake as if you're zipping across the waves or through the sky, and strain into a chilled glass. Grapefruit peel garnish optional.  

If you're looking for a surprisingly pleasant sour drink, the Navigator is it. Gin pairs well with lemon, e.g. the Bee's Knees, and lime, e.g. the Last Word, so there's no reason it wouldn't pair well with grapefruit. The Navigator brings gin together with two citrus flavors. The sugar in the limoncello keeps the Navigator from overpowering you with citrus and botanicals. The flavor balance could come at a price if you're not careful. Overindulge in Navigators and you could end up way off course, both literally and sobrietally. 

To paraphrase the Bible verse, seek a Navigator and you shall find a really good drink.