Sounds more like incompetent gelato maker. It's easy to get it grainy if you overprocess it. I know -- I did it more than a few times when I was doing pastry.

Leave it in the machine too long and yes, it starts to turn into butter. Very unpleasant mouthfeel.
FWIW, one of the ice cream flavors I had to make was sour cream, which while has a lower fat content, is not that far off. And it was fabulous.
If you think of tomato as the fruit it is, you might be more inclined to tomato gelato. I've made tomato sorbet, which needs the best possible in-season tomatoes (plus, I admit, a little goose with best-quality tomato paste).
How would you feel about an olive-oil gelato? This is a real product, served at
Otto, one of Mario Batali's restaurants.