  We have performed resonant X-ray diffraction experiments on the
antiferromagnet GdRu$_{2}$Al$_{10}$ and have clarified that the magnetic
structure in the ordered state is cycloidal with the moments lying in the $bc$
plane and propagating along the $b$ axis. The propagation vector shows a
similar temperature dependence to the magnetic order parameter, which can be
interpreted as being associated with the gap opening in the conduction band and
the resultant change in the magnetic exchange interaction. Although the $S=7/2$
state of Gd is almost isotropic, the moments show slight preferential ordering
along the $b$ axis. The $c$ axis component in the cycloid develops with
decreasing temperature through a tiny transition in the ordered phase. We also
show that the scattering involves the $\sigma$-$\sigma'$ process, which is
forbidden in normal $E1$-$E1$ resonance of magnetic dipole origin. We discuss
the possibility of the $E1$-$E2$ resonance originating from a toroidal moment
due to the lack of inversion symmetry at the Gd site. The spin-flop transition
in a magnetic field is also described in detail.
