The GStopType field indicates whether a feature in the Stops feature class is a regular public transit stop, a parent station containing one or more regular stops, or a station entrance. For a given stop, the ParentID field defines the ID of another feature in the Stops feature class that is the parent station to which the given stop belongs. However, if the GStopType field value is missing, stops features cannot be defined as parent stations, and all features are assumed to be ordinary stops, so the ParentID field will be ignored. For more information, see ArcGIS Network Analyst extension public transit data model.
You can ignore this warning, or you can add a GStopType field to your Stops feature class and populate it with correct values according to the ArcGIS Network Analyst extension public transit data model.