The effect of substitution of Al for Cu on the lattice parameters, and the magnetic and electrical properties of the GdCu 5 - x Al x compounds has been studied. The behaviour of the compounds with low Al concentration (0=<x=<0.5) differs markedly from that of the compounds with high Al concentration (1.0=<x=<1.5): for x=<0.5 the compounds appear to have a complex magnetic structure, whereas for the higher Al concentration a standard two-sublattice model yields values for the intra- and intersublattice coupling parameters. For x=<0.5, below the ordering temperature, the magnetic component of the electrical resistivity decreases with increasing temperature, whereas for the higher Al concentration this component increases. These electrical properties are analysed in terms of a change in the band gap with increasing Al concentration, taking into account spin fluctuation terms.