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Neutron-diffraction investigations have been carried out on a single crystal of $\mathrm{Gd}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ ($\ensuremath{\delta}\ensuremath{\simeq}0.5$) to investigate the magnetic ordering of the Gd and Cu moments at low temperatures. Gd${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6.5}$ orders at ${T}_{N}=2.2$ K with a wave vector (\textonehalf{}, \textonehalf{}, 0). The Gd moments are antiferromagnetically coupled to their nearest neighbors in the $a\ensuremath{-}b$ plane but are ferromagnetically coupled along the crystallographic $c$ axis in contrast to the case of Gd${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ where the coupling along this direction is antiferromagnetic. This result clearly establishes the importance of the oxygen occupancy on the magnetic ordering in $\mathrm{Gd}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ ($0&lt;\ensuremath{\delta}&lt;1$). No evidence for ordering of copper moments was obtained from our present results.

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