A crystallographic study and theoretical assessment of the Gd/Y site preferences in the Gd5−xYxTt4 (Tt = Si, Ge) series prepared by high-temperature methods is presented. All structures for the Gd5−xYxSi4 system belong to the orthorhombic, Gd5Si4-type (space group Pnma). For the Gd5−xYxGe4 system, phases with x < 3.6 and x ≥ 4.4 adopt the orthorhombic, Sm5Ge4-type structure. For the composition range of 3.6 ≤ x ≤ 4.2, a monoclinic, U2Mo3Si4-type structure (space group P21/c) occurs as the majority phase. This structure type has not been previously observed in the RE5T4 (T = Si, Ge, Ga) system and differs from the known monoclinic structure of Gd5Si2Ge2-type (space group P21/a) because all Ge···Ge contacts between slabs are equivalent. The structural relationships between the Sm5Ge4-type and the U2Mo3Si4-type structures are discussed. Single crystal refinements of the occupancies for the three sites for Gd/Y atoms in the asymmetric unit reveal a partially ordered arrangement of Gd and Y atoms. TB-LMTO-ASA calculations were performed to study these atomic distributions as well as to elucidate possible electronic forces that might drive the structural variation. These results illustrate the importance of one of the Gd/Y-sites in shaping the magnetic and structural features observed in Gd5−xYxTt4 system. The magnetic properties of some of the Gd5−xYxTt4 phases are also reported. Germanides with x ≤ 2 show a metamagnetic-type transition similar to Gd5Ge4 from 57−92(2) K. As the Y concentration increases (3 ≤ x ≤ 4), these phases exhibit at least ferrimagnetic ordering with transition temperatures ranging from 15−31(2) K to the paramagnetic state.