We derive Einstein's field equations G_μν + Λg_μν = (8πG/c⁴)T_μν from the elastic substrate dynamics of the U-Cell Model (UCM). The derivation proceeds in four steps. First, the unique stationary, spherically symmetric substrate flow around a mass M is determined by three conditions — stationarity, spherical symmetry, and the exact Bernoulli energy integral of the incompressible irrotational substrate — to be v_0(r) = -c√(2GM/c²r), the free-fall velocity from infinity. This is exact, not a Newtonian approximation, because the substrate is the background medium itself. Second, this flow generates the Gullstrand–Painlevé metric, algebraically equivalent to the Schwarzschild metric and regular at the event horizon. Third, Lovelock's uniqueness theorem (1971), applied to the general dynamical substrate Euler equation for arbitrary mass distributions, uniquely identifies the resulting geometric tensor as G_μν + Λg_μν. Fourth, the Newtonian limit fixes the prefactor to 8πG/c⁴. The gravitational constant is derived from substrate parameters: G = ℏc⁵/(12π E²_Λ,grav). Gravitational waves emerge as transverse-traceless substrate oscillations at c with two polarisations, consistent with GW170817 to one part in 10¹⁵. All quantities that standard GR postulates — the equivalence principle, the metric structure, the Einstein tensor, the prefactor, and the gravitational wave speed — are derived from a single physical picture: spacetime is an elastic substrate, and gravity is its flow.