Bounds on Whittaker Functions for $\mathrm{G}\mathrm{L}(n)$

We prove a new estimate on Jacquet-Whittaker function for ${\mathrm{G}\mathrm{L}}_n(\mathbb{R})$, assuming that the Langlands parameters are purely imaginary and well-spaced. This gives an upper bound for the global sup-norm of the Whittaker function that matches the lower bound of Brumley-Templier in the exponent up to a linear error in $n$.