What is the meaning of gross mass?

It usually refers to the mass of an object together with its container. For example the gross mass of a tin of soup will be the mass of the soup plus the mass of the tin. As a consumer, you will probably be interested in only the net mass - the mass of the soup - but for the haulier it is the gross mass that matters.

For something like breakfast cereal the mass of the container may well be a significant proportion of the gross mass.