Electrical resistivity, magnetic susceptibility, thermal expansion, x-ray diffraction, and scanning calorimetry measurements have been performed over wide temperature ranges on the pseudobinary compounds GdCu1-xGax. In GdCu, which forms in the CsCl crystal structure at room temperature when prepared from the melt, a crystal-structure transformation into a low-temperature noncubic phase occurs at 250 K. Large hysteresis effects are observed with the transformation back into the CsCl structure occurring around 600 K, probably due to a stress-relieving process. In GdCu1-xGax the CsCl crystal structure is stabilized for x0.03. The CsCl pseudobinary compound GdCu1-xGax exhibits a kink-like anomaly in the magnetic susceptibility for 0.03x0.17. This is ascribed to a complicated type of long-range antiferromagnetic ordering. For 0.17<x<0.29 we suggest a mixed or canted low-temperature magnetic state for the ordered Gd moments. Over the whole 0.03x<0.29 concentration region a negative temperature coefficient is observed in the measured electrical resistivity at the ordering temperature. For x0.29 ferromagnetic long-range order is found. A simple model with only nearest-neighbor magnetic interactions, depending on the local Cu and Ga surroundings, is proposed to describe the x dependence of the paramagnetic Curie temperature. Finally a structural and magnetic phase diagram is constructed for the GdCu1-xGax system with 0x0.5.