The sign of the nearest-neighbor exchange interaction ${J}_{1}$ and its logarithmic derivative $\frac{\ensuremath{\partial}\mathrm{ln}{J}_{1}}{\ensuremath{\partial}r}$ has been determined for the intermetallic type-II antiferromagnet GdAs. The ${\mathrm{As}}^{75}$ nuclear-magnetic-resonance Knight-shift data in the paramagnetic state of GdAs were used to yield a value for the Curie-Weiss temperature $\ensuremath{\theta}\ensuremath{\simeq}\ensuremath{-}35\ifmmode^\circ\else\textdegree\fi{}$K. Combining this value with the N\'eel temperature ${T}_{N}\ensuremath{\simeq}25\ifmmode^\circ\else\textdegree\fi{}$K and using the molecular field theory gives ${J}_{1}=\ensuremath{-}0.08\ifmmode^\circ\else\textdegree\fi{}$K and ${J}_{2}=\ensuremath{-}0.40\ifmmode^\circ\else\textdegree\fi{}$K. The measurement of the sign of $\frac{\ensuremath{\partial}\mathrm{ln}{J}_{1}}{\ensuremath{\partial}r}$ in GdAs was performed using low-temperature x-ray powder-diffraction techniques in order to study the exchange-striction-induced trigonal lattice distortion for $T<{T}_{N}$. The observed trigonal distortion was found to be a compression along the [111] direction of the NaCl-type GdAs crystal structure. From the sign of the trigonal distortion, it is concluded that $\frac{\ensuremath{\partial}\mathrm{ln}{J}_{1}}{\ensuremath{\partial}r}$ is negative. Similar data for GdP suggest that $\frac{\ensuremath{\partial}\mathrm{ln}{J}_{1}}{\ensuremath{\partial}r}$ is also negative for this compound.