External stimuli offer a route to realize exotic states with nontrivial excitations in highly frustrated kagome antiferromagnets. Here, 1.5% Gd³⁺ ions are substituted for Y³⁺ ions in YCu₃(OD)_6.5Br_2.5 (abbreviated as YCu₃), yielding Gd_0.015Y_0.985Cu₃(OD)_6.5Br_2.5, where the Gd³⁺ ions act as a local probe to investigate fractionalized excitations in the S = 1/2 kagome Heisenberg antiferromagnet YCu₃. The Gd substitution level is confirmed through detailed x-ray diffraction analyses, while magnetization and magnetic-susceptibility measurements disclose the interaction between the substituted Gd ions and the Cu-spin kagome network. Specific-heat measurements in magnetic fields up to 14 T and down to dilution-refrigerator temperatures reveal an enhanced density of low-energy states, indicative of spinon-impurity coupling, yet higher-energy excitations are hardly affected. Furthermore, high-frequency electron spin resonance (ESR) measurements, enabled by the presence of Gd as a local probe, present the resonance associated with Gd ions coupled with the kagome spins. In particular, the temperature dependence of the ESR linewidth exhibits a near-quadratic power-law behavior, revealing the presence of Dirac spinon dynamics in the finite-temperature regime above 2 K.