GeometryCollectionComponent has a SetRestCollection function but does not expose it to blueprints.
Unfortunately, I think it needs to be handled in C++.
If you are using the source code version of UE, you only need to add a UFUNCTION macro to the SetRestCollection function in GeometryCollectionComponent.h.
If you are using the binary version, you will need to add a C++
class and create a function that assigns an asset reference to the RestCollection variable.