I have seen a topic on this before, dont know if they ever found a solution but much help is needed.
my unity gestures section is gone in the playable layers , ive tried making a whole new project and removed the Sdk from the old one and just reimported and same thing happened, Ive also looked through every folder to see what would cause the issue.
You should post an image of this part of the avatar descriptor. I have seen once where the playable layers listed in the avatar descriptor were wrong, specifically two fx layers, and something missing.
Fix for that was to note down which layers were used, just in case the project has some extras, then clear the playable layers. Back to defaults? There is a button you can click to make the layers all disappear, then when you click the button to show the layers, they should all be there properly. Just need to reassign.