Gd3NSe2O crystallizes in the monoclinic C2/m space group. There are three inequivalent Gd sites. In the first Gd site, Gd(1) is bonded in a 7-coordinate geometry to two equivalent N(1), three equivalent Se(1), and two equivalent O(1) atoms. In the second Gd site, Gd(2) is bonded in a 6-coordinate geometry to one N(1), one Se(1), two equivalent Se(2), and two equivalent O(1) atoms. In the third Gd site, Gd(3) is bonded to one N(1), two equivalent Se(1), and three equivalent Se(2) atoms to form distorted edge-sharing GdSe5N octahedra. N(1) is bonded to one Gd(2), one Gd(3), and two equivalent Gd(1) atoms to form NGd4 tetrahedra that share corners with three equivalent Se(1)Gd6 octahedra, corners with five equivalent Se(2)Gd5 square pyramids, corners with two equivalent N(1)Gd4 tetrahedra, corners with two equivalent O(1)Gd4 tetrahedra, edges with three equivalent Se(1)Gd6 octahedra, and edges with two equivalent O(1)Gd4 tetrahedra. The corner-sharing octahedral tilt angles range from 28-40°. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded to one Gd(2), two equivalent Gd(3), and three equivalent Gd(1) atoms to form distorted SeGd6 octahedra that share corners with two equivalent Se(2)Gd5 square pyramids, corners with three equivalent N(1)Gd4 tetrahedra, corners with four equivalent O(1)Gd4 tetrahedra, edges with four equivalent Se(1)Gd6 octahedra, edges with three equivalent Se(2)Gd5 square pyramids, edges with two equivalent O(1)Gd4 tetrahedra, and edges with three equivalent N(1)Gd4 tetrahedra. In the second Se site, Se(2) is bonded to two equivalent Gd(2) and three equivalent Gd(3) atoms to form SeGd5 square pyramids that share corners with two equivalent Se(1)Gd6 octahedra, corners with two equivalent O(1)Gd4 tetrahedra, corners with five equivalent N(1)Gd4 tetrahedra, edges with three equivalent Se(1)Gd6 octahedra, edges with four equivalent Se(2)Gd5 square pyramids, and  an edgeedge with one O(1)Gd4 tetrahedra. The corner-sharing octahedral tilt angles are 9°. O(1) is bonded to two equivalent Gd(1) and two equivalent Gd(2) atoms to form OGd4 tetrahedra that share corners with four equivalent Se(1)Gd6 octahedra, corners with two equivalent Se(2)Gd5 square pyramids, corners with two equivalent N(1)Gd4 tetrahedra, corners with two equivalent O(1)Gd4 tetrahedra, edges with two equivalent Se(1)Gd6 octahedra,  an edgeedge with one Se(2)Gd5 square pyramid,  an edgeedge with one O(1)Gd4 tetrahedra, and edges with two equivalent N(1)Gd4 tetrahedra. The corner-sharing octahedral tilt angles range from 36-50°.
Gd3NSe2O crystallizes in the monoclinic C2/m space group. There are three inequivalent Gd sites. In the first Gd site, Gd(1) is bonded in a 7-coordinate geometry to two equivalent N(1), three equivalent Se(1), and two equivalent O(1) atoms. Both Gd(1)-N(1) bond lengths are 2.37 Å. There is one shorter (3.08 Å) and two longer (3.12 Å) Gd(1)-Se(1) bond lengths. There is one shorter (2.31 Å) and one longer (2.36 Å) Gd(1)-O(1) bond length. In the second Gd site, Gd(2) is bonded in a 6-coordinate geometry to one N(1), one Se(1), two equivalent Se(2), and two equivalent O(1) atoms. The Gd(2)-N(1) bond length is 2.25 Å. The Gd(2)-Se(1) bond length is 2.98 Å. Both Gd(2)-Se(2) bond lengths are 3.06 Å. Both Gd(2)-O(1) bond lengths are 2.31 Å. In the third Gd site, Gd(3) is bonded to one N(1), two equivalent Se(1), and three equivalent Se(2) atoms to form distorted edge-sharing GdSe5N octahedra. The Gd(3)-N(1) bond length is 2.19 Å. Both Gd(3)-Se(1) bond lengths are 2.88 Å. There are two shorter (2.91 Å) and one longer (3.26 Å) Gd(3)-Se(2) bond length. N(1) is bonded to one Gd(2), one Gd(3), and two equivalent Gd(1) atoms to form NGd4 tetrahedra that share corners with three equivalent Se(1)Gd6 octahedra, corners with five equivalent Se(2)Gd5 square pyramids, corners with two equivalent N(1)Gd4 tetrahedra, corners with two equivalent O(1)Gd4 tetrahedra, edges with three equivalent Se(1)Gd6 octahedra, and edges with two equivalent O(1)Gd4 tetrahedra. The corner-sharing octahedral tilt angles range from 28-40°. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded to one Gd(2), two equivalent Gd(3), and three equivalent Gd(1) atoms to form distorted SeGd6 octahedra that share corners with two equivalent Se(2)Gd5 square pyramids, corners with three equivalent N(1)Gd4 tetrahedra, corners with four equivalent O(1)Gd4 tetrahedra, edges with four equivalent Se(1)Gd6 octahedra, edges with three equivalent Se(2)Gd5 square pyramids, edges with two equivalent O(1)Gd4 tetrahedra, and edges with three equivalent N(1)Gd4 tetrahedra. In the second Se site, Se(2) is bonded to two equivalent Gd(2) and three equivalent Gd(3) atoms to form SeGd5 square pyramids that share corners with two equivalent Se(1)Gd6 octahedra, corners with two equivalent O(1)Gd4 tetrahedra, corners with five equivalent N(1)Gd4 tetrahedra, edges with three equivalent Se(1)Gd6 octahedra, edges with four equivalent Se(2)Gd5 square pyramids, and  an edgeedge with one O(1)Gd4 tetrahedra. The corner-sharing octahedral tilt angles are 9°. O(1) is bonded to two equivalent Gd(1) and two equivalent Gd(2) atoms to form OGd4 tetrahedra that share corners with four equivalent Se(1)Gd6 octahedra, corners with two equivalent Se(2)Gd5 square pyramids, corners with two equivalent N(1)Gd4 tetrahedra, corners with two equivalent O(1)Gd4 tetrahedra, edges with two equivalent Se(1)Gd6 octahedra,  an edgeedge with one Se(2)Gd5 square pyramid,  an edgeedge with one O(1)Gd4 tetrahedra, and edges with two equivalent N(1)Gd4 tetrahedra. The corner-sharing octahedral tilt angles range from 36-50°.