I would expect this to work:
... "Hooks": { ... "LoadExtensionSchemaUpdates": [ "Gather\\UpdaterHooks::onLoadExtensionSchemaUpdates", "Gather\\UpdaterHooks::onLoadExtensionSchemaUpdatesBC" ] }
but actually it will call Gather\UpdaterHooks::onLoadExtensionSchemaUpdates with the string "Gather\\UpdaterHooks::onLoadExtensionSchemaUpdatesBC" as the first parameter.