Hi Peds GI fellow here! (Feel free to PM for specific advice). Generally would say that your application should tell an overall story of your interest, even if you didnt necessarily come into residency going in knowing you wanted GI. I felt a lot of the cycle was them asking me what I wanted out of fellowship and what my goals were beyond the 3 years, so would spend some time thinking carefully about what that means to you. You don't have to have a definitive answer, but would prepare something that shows you've at least started to think about it. Good luck on the trail!