Pour 2 liters of milk into the Instant Pot, turn it on on "sauté" or "boil" (turn on "boil" by pressing the "yogurt" button twice) mode. Allow the milk to warm to 180 degrees F, whisking often. Use a food thermometer to test the temperature. Turn off the Instant Pot by pressing "cancel".
If you want to add the gelatin powder to your batch of yogurt, this is moment to do so. Pour about 1 cup of hot milk into a separate container and sprinkle 1 tablespoon of gelatin on top. Whisk well until the gelatin powder is completely dissolved. Add the milk back into the Instant Pot and stir together.
Let the milk cool off to about 105-115 degrees F. To speed up the process, you can place the stainless steel pot in your sink filled with cold water. Whisk periodically and check the temperature with the food thermometer.
Once the milk is cooled, it's time to add the starter culture (or yogurt from previous batch). Take about 1 cup of milk out, add 1/3 teaspoon of your starter culture or 1/2 cup of yogurt, stir well until dissolved and then add back to the pot and stir together with the rest of the milk.
Put the pot back into the Instant Pot, place the lid on, and press the "yogurt" button. Set the timer to your desired incubation time (from 8 up to 24 hours). I usually incubate my yogurt for 15 hours and I find I like it the best.
Once the incubation time is up, remove the pot and place it in the refrigerator. Let the yogurt set for at least 2 hours (or more if you added the gelatin). After this time, you can enjoy your yogurt, or strain it over a colander lined with a butter cloth to make it thicker (Greek yogurt-like).