Because I couldn’t find a pattern on Ravelry that made just the grapefruit I envisioned, I created my own using Amy Szczepanski’s method of generating a knitting pattern for any surface of revolution (published in her chapter of Crafting by Concepts).
The finished grapefruit is about 5” in diameter.
Any DK-weight yarn will do for the body of the grapefruit, and any DK-or-heavier weight yarn will work for the stem.
The only stitches needed are: knit, slip, make 1, knit two together.
There are notes included on the mathematics used in creating the pattern.