  We study the magnetism in NdFe$_2$Ga$_8$ by the neutron-diffraction and
temperature-modulated linear {Gr\"uneisen} parameter measurements. Previous
thermodynamical measurements have demonstrated that there are two magnetic
transitions at 10 and 14.5 K, respectively. Neutron-diffraction measurements
confirm that the lower one is an antiferromagnetic (AFM) transition with a
commensurate magnetic structure. Both the commensurate and the incommensurate
(IC) magnetic peaks are found below the higher transition but their intensities
only gradually increase with decreasing temperature. Below 10 K, the
commensurate peak intensity increases quickly with decreasing temperature,
signaling the AFM transition, while the IC peak intensity disappears below 5 K.
The linear {Gr\"uneisen} parameter along the $c$ axis, $\Gamma_c$, shows a
hysteresis behavior that is different from the hysteresis behavior for the
magnetization $M$. We give a discussion of the origin of the magnetism in
NdFe$_2$Ga$_8$.
