I just encounter this issue and figured out how to fix it.
I had created a map with several mods that it did not need to depend on (accidentally) and wanted to remove them because I though one of the mod dependencies was causing this problem with missing great people. While it was related it was not the cause. I have made custom maps before in world builder at higher sizes using the Larger Map Sizes Mod in the past that had working great people, but my most recent one did not have great people available. As a test I made a quick bare bones map with minimum mods but at the same size as my flawed map randomly generated in world build and let it generate a map mod to use in a custom game. I created an advanced custom single player game in this test map and great people showed up as available on turn 1, whereas in the flawed map all GP slots said 'everyone of this type has been claimed' (or a vaguely similar statement)
I followed the instructions on this post:
https://www.reddit.com/r/civ/comments/c78in3/in_world_builder_is_there_a_way_to_remove_mod/
To view the world builder save file of my flawed map and compare it to the version that was correct.
This is the Map table of my flawed map.
This is the map table for the working worldbuilder save map
All I had to do was put a valid value in the MapSizeType column and started a new game from my flawed map and it worked, I had all GP possibilities listed on turn one. Note that I also tried to reload my flawed map into Worldbuilder and resave, but the cell ended up blank again, so next time I edited the blank cell I did not reopen it worldbuilder before I created a new custom game off the corrected map.
I do not know if this fix will correct game saves that have missing GP, probably not. But you can fix it if your willing to start over a game and it can salvage maps you might have otherwise though had become irretrievably broken, if you didn't want to play without GP.