The substitution of Gd for Tm in the pseudobinary system gives rise to the onset of long-range magnetic order in the itinerant Co sublattice above a critical concentration . The features of the Co magnetism around the critical concentration were studied by means of resistivity measurements in magnetic fields up to 10 T and pressures up to 15 kbar. An extremely large drop of the resistivity at the ordering temperature measured for is explained by a suppression of the spin-fluctuation contribution to the resistivity in the vicinity of due to the intersublattice molecular field. The analysis of the magnetoresistance revealed that the spin fluctuations above are additionally enhanced due to short-range correlations within the f - d-electron system in a limited temperature region above . The application of external pressure results in a suppression of both the spin fluctuations and the onset of the long-range order in the Co sublattice. In the Co sublattice remains paramagnetic below the Curie temperature for . The magnetic Grüneisen parameter was found to substantially increase as the critical concentration was approached.