Gauge-Invariant Separability, Trace Anomaly, and the Yang–Mills Mass Gap The Yang–Mills Mass Gap problem stands as one of the central unresolved questions in mathematical physics. Despite overwhelming numerical and experimental evidence for a positive mass gap in four-dimensional non-Abelian gauge theories, a rigorous derivation within a fully defined quantum field theoretic framework has remained elusive. The Clay Millennium formulation separates the problem into two logically distinct components: the construction of a mathematically consistent four-dimensional Yang–Mills quantum field theory, and the demonstration that its physical Hamiltonian spectrum exhibits a strictly positive gap above the vacuum. The present work addresses the second component under the explicit assumption that the first has been resolved. Rather than attempting to derive the mass gap through confinement mechanisms, symmetry breaking, thermodynamic arguments, or phenomenological modeling, we show that the gap follows necessarily from structural properties already implicit in any Yang–Mills theory satisfying the Osterwalder–Schrader or Wightman axioms. In this sense, the mass gap is not treated as a contingent dynamical feature, but as a consequence of gauge-invariant state structure and quantum consistency. Our approach isolates three ingredients that are unavoidable in such a theory: (i) the restriction of physical excitations to gauge-invariant local operator algebras, (ii) the positivity properties of modular Hamiltonians and relative entropy for localized states, and (iii) the emergence of a nonzero renormalization-group scale through the trace anomaly and dimensional transmutation. When combined, these elements enforce a coercive lower bound on the energy of any non-vacuum physical excitation. A central point of emphasis is that gauge symmetry remains exact throughout. Gauge-variant fields play no direct role in the physical Hilbert space, and no assumption is made regarding confinement or the behavior of Wilson loops at large distances. The argument operates entirely at the level of local, gauge-invariant observables and their associated state spaces. In particular, the mass gap arises independently of any infrared conjecture about color charge screening or string formation. The result may be viewed as establishing Part B of the Clay Millennium Problem conditionally on Part A: once a four-dimensional Yang–Mills quantum field theory satisfying the standard axioms exists, a positive mass gap is unavoidable. This reframes the open problem by clarifying that the remaining difficulty lies entirely in the construction of the theory itself, not in the spectral properties of its physical Hamiltonian.