Closest primary slot in range can either be filled in by you any way that you like, or you can use the helper static function node in VRExpansionFunctionLibrary::GetGripSlotInRangeByTypeName and pass in the prefix for the slots you are looking for.
My Default implementation in the grippable components designates primary slots as “VRGripP” and then a number and secondary slots as “VRGripS” and then a number. The helper function searches through all slots on the mesh and checks ranges for the ones that start with the correct prefix.
There are only default implementations on the grippable objects though, any custom class that doesn’t derive from one of those needs to fill out the function itself. You can also override the default in any of the grippable objects in blueprint by doing what you are doing.