To insert the Gravity Point or to calculate the mass of an Assembly, all solids in the Assembly must have a material with an assigned density parameter.
By using Exclude from GP parameter, you can make an exception to that.
All elements that have this parameter turned ON will not be included when calculating the Assembly mass or center of gravity.
In this example, the grout tube for a precast wall has this parameter active, so the Assembly Mass and Element mass parameters are empty:
Other families, however, were included in the calculation of the assembly mass and center of gravity.