The structural and magnetic characteristics of -based ternaries, in which Co is partly substituted by Ga atoms, have been investigated by means of x-ray powder diffraction (XRPD) and magnetic measurements. XRPD patterns show that all samples are single phase with the rhombohedral -type structure except the sample with x = 8 which contains a small amount of hcp Co and rare-earth-rich phase as impurity phases. The Ga substitution for Co in these compounds leads to a monotonic increase in unit cell volume and a monotonic decrease in Curie temperature. The saturation magnetization is found to approximately linearly decrease with increasing Ga concentration. For compounds with x = 4, 5 and 6, compensation points are observed in the corresponding temperature dependence of the magnetization. The values of compensation temperature are found to shift to higher temperature with increasing Ga content. The intersublattice-molecular-field coefficients, , have been determined on the basis of magnetization curves measured at the compensation temperature. The exchange-coupling constants have been calculated by analysing the field dependence of the magnetization and by using the method based on magnetic-ordering temperatures. It has been found that the Ga substitution for Co in these compounds has little influence on Er-Co exchange interaction. XRPD patterns on magnetically aligned samples with show that the easy magnetization direction at room temperature corresponds to the c-axis.