Gravitational Admissibility as a Calibration Case: Exact Reduction to Domain-Relative Causal Reachability in General Relativity

General relativity already determines causal reachability once a spacetime geometry and target domain are specified. This paper uses that fact as a calibration case for admissibility-based structural vocabularies in physics. For an event p and declared observational domain O, gravitational admissibility is defined operationally by whether a future-directed causal trajectory from p reaches O. The corresponding binary indicator reduces to ordinary causal structure: a_O(p)=1 if and only if p lies in J^-(O), with the conformal completion fixed when O is a conformal boundary such as future null infinity. The reduction is therefore not presented as new gravitational physics. Instead, it is used as an eliminability-based methodological benchmark: when a base theory already determines the relevant support relation, an additional admissibility layer should be physically conservative and eliminable unless it earns a separate organizational role. The framework is illustrated across black-hole, cosmological, and Rindler horizons. The Rindler case provides a flat-spacetime control demonstrating that reachability closure can arise from observer-relative causal structure without local spacetime curvature or an additional force. The paper also distinguishes binary reachability from optional observer-dependent null escape-cone diagnostics, separates null, timelike-geodesic, and accelerated causal trajectory classes, and clarifies the boundary between classical causal reachability and quantum-field-theoretic phenomena such as the Unruh and Hawking effects. The broader methodological proposal is a calibration test for candidate structural vocabularies: identify the admissible object, declare the target or context, specify the independently given dynamics, ask whether the proposed admissibility quantity is derivable from those dynamics, and require any retained terminology to demonstrate a genuine organizational benefit. In general relativity, the binary admissibility layer collapses exactly into standard causal structure, making GR a useful control case for evaluating stronger admissibility claims elsewhere.