In this paper, we present a combinatorial proof of the Gaussian product inequality (GPI) conjecture in all dimensions when the components of the centered Gaussian vector X = (X 1 , X 2 , . . . , X d ) can be written as linear combinations, with nonnegative coefficients, of the components of a standard Gaussian vector. The proof comes down to the monotonicity of a certain ratio of gamma functions. We also show that our condition is weaker than assuming the vector of absolute values |X| := (|X 1 |, |X 2 |, . . . , |X d |) to be in the multivariate totally positive of order 2 (MTP 2 ) class on [0, ∞) d , for which the conjecture is already known to be true.