We present a unified study of scalar, vector, and tensor quasinormal modes (QNMs) of Schwarzschild black holes corrected by a Gauss-Bonnet (GB) term in higher dimensions. Using a high-precision Chebyshev spectral method, we map the QNM spectra across $D\in \{5,6,7,8,10,11,12,26\}$ well beyond the regime where sixth-order Wentzel-Kramers-Brillouin and characteristic-integration techniques remain reliable. Across the three spin sectors, we find several robust signatures of higher-curvature dynamics: the appearance of overdamped purely imaginary modes, nonmonotonic behavior in the real parts of higher overtones, and a strong amplification of the dimensionless QNM frequencies in string-motivated dimensions. In the scalar and vector sectors, we uncover an exact isospectrality between the scalar monopole ($\ell =0$) and vector dipole ($\ell =1$) at vanishing GB coupling, and we provide an analytic proof based on a Darboux factorization of the corresponding Hamiltonians. In the tensor sector, we obtain the first numerical confirmation of the long-predicted instability in six dimensions; its onset is sharply captured by the Cohn-Calogero bound and leads to the mass threshold $GM\le 158.1{\alpha }^{3/2}$. No analogous instability is found for $D\ge 7$, and no tensor isospectrality occurs. Converting the dimensionless frequencies to physical units suggests that the amplified modes in higher dimensions may enter the sensitivity window of future space-based detectors such as DECIGO. The merged analysis provides a comprehensive benchmark for QNMs in Einstein-Gauss-Bonnet gravity and highlights the limitations of standard approximation schemes in the strong-coupling and high-overtone regimes.

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• Alternative gravity theories
• Gravitational waves
• Gravity in dimensions other than four
• Quantum gravity