To address the macro‑micro dichotomy between general relativity and quantum field theory, this paper constructs a unified gravity model based on a spacetime monistic ontology: the rest mass of matter is not an independent entity but a steady‑state condensation product of continuous local spacetime stacking. The model employs equivalent spacetime density and spacetime pressure as core physical quantities, decomposing the total equivalent density into five linearly superposed components: vacuum background density, gravitational equivalent density, relativistic kinematic equivalent density, quantum fluctuation density, and stacking‑rate additional density, without requiring a separate rest mass density term. Through rigorous relativistic dynamics, dimensional analysis, and logical self‑consistency optimization, the force expression is corrected to a volume‑dependent form, the dimension of the pressure constant K is fixed as [L]²[T]⁻², and the correct relativistic kinetic energy formula is used to recover the classical limit. The linear pressure‑density relation P = K ρ_Σ is currently a phenomenological assumption. This paper provides the calibrated values of all phenomenological parameters: c₀ = 299792458.0166 m·s⁻¹, ρ_c = 1.8×10³⁰ kg·m⁻³, ρ_vac = 9.9×10⁻²⁷ kg·m⁻³, K = 1.47×10⁴ m²·s⁻², determined solely from GPS gravitational time delay and Casimir force experiments. Multi‑scale quantitative verification shows that on macroscopic scales the model’s predictions for the gravitational redshift of Sirius B, the relativistic muon lifetime, and the orbital precession of the Hulse‑Taylor binary pulsar differ from general relativity and observations by less than 5×10⁻⁴; on microscopic scales, by introducing a single fitting constant η ≈ 2.3×10⁻³, the model simultaneously describes the α‑decay half‑lives of U‑238, Pu‑239, and Ra‑226 to within 0.045% accuracy, and naturally yields the Heisenberg uncertainty principle. The model provides a unified explanation of the vacuum speed of light, the four fundamental interactions, relativistic spacetime effects, black hole holography, and the superluminal expansion of the universe, and proposes eight falsifiable quantitative experimental predictions. This paper further proposes a photon‑stacking unification interpretation, which expresses the five densities explicitly in terms of a stacking height field h(x,t) and its derivatives, achieving complete mathematical unification. Version 2 Updates (2026-06-09)This paper further proposes a photon‑stacking unification interpretation, which expresses the five densities explicitly in terms of a stacking height field h(x,t) and its derivatives, achieving complete mathematical unification.Corrected minor typos in symbols and references.Improved clarity in the discussion of multi-scale verification.