What is the covariance matrix, Cov[, b], of the GLS estimator = (X' -1 X)

What is the covariance matrix, Cov[β̂, β̂ −b], of the GLS estimator β̂ = (X'Ω^-1 X)⁻¹ X'Ω⁻¹ y and the difference between it and the OLS estimator, b = (X'X)⁻¹X'y? The result plays a pivotal role in the development of specification tests in Hausman (1978).