Why the heck is IGameplayTagAssetInterface NOT exposed to Blueprints...

[BlueMind_Studio]

You need to have your BP Actor inherit from a C++ Actor with:

#include "GameplayTagAssetInterface.h"
class AFooCharacter : public ACharacter, public IGameplayTagAssetInterface

and add

UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = "GameplayTags")
FGameplayTagContainer GameplayTags;

virtual void GetOwnedGameplayTags(FGameplayTagContainer& TagContainer) const override { TagContainer = GameplayTags; return; }

also extend your build.cs:

PublicDependencyModuleNames.AddRange(new string]{ "Core", ....,  "**GameplayTags**" });