From what I can tell, for any object that is not located in a specifically placed physics volume, it falls back to the “DefaultPhysicsVolumeClass” that is set in the World Settings.
So, I believe if you wanted a global gravity change outside of placed physics volumes, you’d want to potentially add your own DefaultPhysicsVolumeClass that overrides the GetGravityZ function, where you’d apply your gravity changes at runtime.
I haven’t tried this myself, so not sure if that will work or if it can be done with blueprints alone. Looks like it might require some C++ work to expose ADefaultPhysicsVolume as a BlueprintType, or might be easier at that point to just derive your own DefaultPhysicsVolume in code (or just add a GetGravityZ override directly to ADefaultPhysicsVolume) and expose gravity scale properties used in the overridden GetGravityZ that can be tweaked via blueprint as needed.