We derive new results on the characterization of Gelfand-Shilov spaces Sνμ(Rn), μ, ν > 0, μ + ν ≥ 1 byGevrey estimates of the L2 norms of iterates of (m, k) anisotropic globally elliptic Shubin (or Γ) type operators, (- Δ)m/2 + |x>k with m, k ∈ 2ℕ being a model operator, and on the decay of the Fourier coefficients in the related eigenfunction expansions. Similar results are obtained for the spaces Σνμ(Rn), μ, ν > 0, μ + ν > 1, cf. (1.2). In contrast to the symmetric case μ = ν and k = m (classical Shubin operators) we encounter resonance type phenomena involving the ratio κ:= μ/ν; namely we obtain a characterization of Sνμ(Rn) and Σνμ(Rn) in the case μ = kt/(k + m), ν = mt/(k + m), t ≥ 1, that is, when κ = k/m ∈ ℚ. © 2019, The Hebrew University of Jerusalem.