Exotic magnetism on the quasi-FCC lattices of the d$^{3}$ double perovskites La$_{2}$NaB'O$_{6}$ (B' $=$ Ru, Os)

B-site ordered double perovskites with quantum spins S $=$ 1/2 (d$^{1})$ and S $=$ 1 (d$^{2})$ on the B' site have attracted a great deal of recent interest, due to the possibility of studying 4d and 5d magnetism combined with magnetic frustration on the face-centered-cubic (FCC) lattice. There has been less focus on d$^{3}$ systems, as they are generally expected to behave more classically and yield simple, commensurate magnetic ground states. In contrast, we find evidence for long-range and short-range ($\xi =$ 70 {\AA} at 4 K) incommensurate magnetic order on the quasi-FCC lattices of the monoclinic double perovskites La$_{2}$NaRuO$_{6}$ and La$_{2}$NaOsO$_{6}$ respectively. Incommensurate magnetic order on the FCC lattice has not been predicted by mean field theory, but may arise via a delicate balance of inequivalent nearest neighbor and next nearest neighbor exchange interactions. Furthermore, in the Ru system with long-range order, inelastic neutron scattering reveals a spin gap $\Delta $ $=$ 2.75 meV. Magnetic anisotropy is generally minimized in the more familiar octahedrally-coordinated 3d$^{3}$ systems, so the large gap observed for La$_{2}$NaRuO$_{6}$ may result from the significantly enhanced value of spin-orbit coupling in this 4d$^{3}$ material.