We study Sibley's guard-point convexity measure for simple polygons and compare it with the exterior and perimeter convexity measures. We prove the exterior inequality G(F) <= E(F) and disprove the pointwise perimeter inequality G(F) <= P(F) by an explicit nonconvex pentagon with G(F) = 62/63 and P(F) = 185/189. Nevertheless, we prove the uniform bound G(F) <= 2P(F) for every simple polygon. Thus the pointwise perimeter inequality is false, but the corresponding asymptotic non-domination conclusion remains true. We also record an auxiliary guard-point-adapted anisotropic perimeter ratio, which isolates the directional loss in the Euclidean perimeter comparison.

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