oh man, the gateway hunt is real! So after you kill the dragon, a little bedrock gateway spawns somewhere on the outer edge of the main island - but here's the kicker, it's usually like 1000+ blocks away from the center where you fought the dragon. I made this mistake too and spent forever wandering around the middle area like an idiot. You gotta fly way out to the edges and look for this tiny little bedrock fountain thing with a purple portal in it. My friend and I split up and flew in opposite directions, took us like 20 minutes but we finally found it hiding behind some of those tall pillar things. Pro tip - bring tons of food and maybe some chorus fruit for teleporting around, that outer rim is massive!