Looking at your post history, you're a US MD with good exam scores who ? was applying to GI but I assume did not get a spot.
If you've changed your mind and decided you want to do Anesthesia, you would need to submit an ERAS application and try to get a spot in the match. You can start as a PGY-2 -- so you can apply for Advanced or Reserved spots. You could also contact Anesthesia programs about off cycle openings at the PGY-2 level. You would want to do anesthesia rotations in the rest of your R3 to gain experience, LORs, and ensure it's really what you think it might be.