I think the first time I killed the dragon was in 20w20b.  No End Gateway was created, so I built a thousand-block bridge to the islands so I could explore the End Cities.  I did eventually find one End Gateway near an End City.  (I threw in a pearl and was transported to the original obsidian platform near 0,0)

Just today I summoned the dragon for the 2nd time (to get the Advancement), and after killing the dragon, once again, no new End Gateway spawned (at least that I can see, flying around with my elytra).  I made a copy of my survival map and flew around in creative mode, and still couldn't find any End Gatways within 150 blocks of 0,0.