[en] The uniform spin susceptibility, χ_s , is determined from the Gd³⁺ high frequency ESR Knight shift in the underdoped high temperature superconductor, Gd:YBa₂ Cu₄O ₈ (T_c=82 K) between 8 to 260K. Measurements at two fields (5.4 and 8.1T) permit high precision at low temperatures. In the range of 0.1T_c to 0.4T_c , a linear temperature dependence of χ_s is observed with a large slope. The corresponding maximum d-wave superconducting gap is Δ₀=190 K , 1.1 times the weakly coupled d-wave limit. The results support suggestions that the normal state gap observed below 180K and the superconducting gap below T_c have nodes with the same symmetry. copyright 1997 The American Physical Society