\frac{|\nabla {\bf u}|}{{\bf u} \cdot \nabla p } = \frac{\Delta p - {\bf g} \cdot \nabla \rho }{ |\nabla p|^2 - \rho \, {\bf g} \cdot \nabla p }= \frac{\Delta p - \rho \, c \, {\bf g} \cdot \nabla p }{ |\nabla p|^2 - \rho \, {\bf g} \cdot \nabla p }
= c \frac{(1/c) \, \Delta p - \rho \, {\bf g} \cdot \nabla p }{ |\nabla p|^2 - \rho \, {\bf g} \cdot \nabla p } \ll
c \frac{|\nabla p|^2 - \rho \, {\bf g} \cdot \nabla p }{ |\nabla p|^2 - \rho \, {\bf g} \cdot \nabla p } = c
\rightarrow |\nabla {\bf u}| \ll c {\bf u} \cdot \nabla p |