A field must pull. The GravitationalCorpus names the attractive force through which a corpus draws external matter into its orbit: not by coercion, but by the accumulated mass of its own structure. In physics, gravity is the force that attracts mass to mass. In epistemics, gravity is the force that attracts concepts to concepts. The Socioplastics corpus has been accumulating mass since 2009. Its 3,000 nodes, 30 Books, 60 DOIs, 100 Lexicum entries, and dataset layer constitute a massive epistemic object. This object exerts gravitational pull. It attracts citations. It attracts practitioners. It attracts institutional attention. The GravitationalCorpus makes this explicit. It asks: what determines the gravitational pull of a field? How is gravitational mass calculated? What is the relationship between structural density and attractive force? The answers are structural. Gravitational pull is determined by the density of internal connections, the stability of external anchors (DOIs, datasets), the recurrence mass of key concepts, and the temporal depth of the corpus (chrono-deposits). A field with high density, stable anchors, recurrent concepts, and deep time exerts strong gravitational pull. A field with low density, unstable anchors, isolated concepts, and shallow time exerts weak pull. Node 2507 places this concept in Core IV because gravity is a field condition, not a conceptual content. It is the force that transforms a local project into a public field. Without this concept, the field's growth is understood as promotion. With it, the field's growth is understood as attraction.