The giant covalent molecule (GCM) in your context is a molecule ( 3D for diamonds, 2D for graphite ), which size is more or less (breaks are possible) limited just by the size of the particular solid phase region.
If sucrose molecules were interconnected by covalent bonds forming in the ideal case a single molecular structure of the size of the whole crystal, than it would be a GCM. But they are not. Sucrose crystals are formed by independent molecules. ( Table salt is a ionic compound, it does not form covalent molecules.)
Wikipedia.org - Covalent_bond -Covalent_structures says:
There are several types of structures for covalent substances, including individual molecules, molecular structures, macromolecular structures and giant covalent structures.
Network covalent structures (or giant covalent structures) contain large numbers of atoms linked in sheets (such as graphite), or 3-dimensional structures (such as diamond and quartz). These substances have high melting and boiling points, are frequently brittle, and tend to have high electrical resistivity. Elements that have high electronegativity, and the ability to form three or four electron pair bonds, often form such large macromolecular structures.