Quick answer: Unreal Gameplay Tags added to a DataTable don't appear in pickers after reimport? Tag table requires a manual refresh - call FGameplayTagsManager::Get().LoadGameplayTagTables.
Added new tags to GameplayTags.csv and reimported. Tag picker in blueprints still shows the old set.
Force a table reload
In an editor utility blueprint: call Editor Reload Gameplay Tags. Picker refreshes immediately. Otherwise you wait for the next editor restart.
Restart the editor
The blunt fix. Editor caches the tag tree at startup; reimport doesn't invalidate the cache without the manual reload.
Use the tag editor UI
Project Settings > Gameplay Tags > Manage Gameplay Tags. Add via the UI and the cache invalidates automatically.
“Gameplay Tags are loaded once. Adding more requires telling the editor explicitly.”
For tag-heavy projects, prefer the in-editor tag manager over CSV. Source-control conflicts are easier to resolve in INI than CSV anyway.