What is the critical factor differentiating a Super-Earth like Kepler-452b from a gas giant?

Its calculated density, confirming high mass combined with high density

The crucial determination between classifying a massive world like a Super-Earth, such as Kepler-452b, as rocky versus atmosphere-dominated hinges entirely on density measurements derived from mass and volume data. If a planet possesses a high mass but exhibits a low calculated density, this strongly suggests the object is dominated by a large, puffy atmosphere composed largely of lighter elements like hydrogen and helium. Conversely, when a world has a high mass coupled with a high calculated density—indicating that the bulk of its volume is packed with heavy silicates and metals—this confirms a fundamentally large, solid, rocky structure, regardless of its significant mass.