Add sliced, frozen strawberries, bananas, milk, sweetener and gelatin (if using) to a food processor or a high-speed blender and mix / blend until a creamy "soft serve" is formed, scraping down sides as needed - about 3-5 minutes.
Serve immediately or transfer to serving bowls and put in the freezer to thicken up. Serve as is or top with any favorite toppings!
Best when fresh. Leftovers will keep in the freezer up to 1 week. Let it sit out of the freezer and come back to soft serve consistency before enjoying.