Some parameters are not exposed to the filters. In recent versions they have added more parameters but still not all.
If you realy need this functionality you could add a project parameter to pipes (e.g PipeSlope). Then write a Dynamo script to copy the slope values to the new project parameter and then use this new parameter to filter. You will need to run this script again when pipes are added or slope values change.
Louis
Please mention Revit version, especially when uploading Revit files.