I just defeated the dragon in my survival world, but no gateway generated. I went in a circle 96 blocks from 0,0 at Y=75 (like Gamepedia suggested), but couldn't find any. I decided to look at the level.dat file, and it said that there is no gateway generated. I attached an image of it below.
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Thank you for your report!
However, this issue is a Duplicate of MC-184482.
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