We present a systematic investigation into the magnetic anisotropy of triangular-lattice KBaGd(BO3)2, a high- performance magnetic refrigeration candidate, utilizing magnetic characterization and electron spin resonance (ESR) techniques. KBaGd(BO3)2 exhibits an easy-plane magnetic anisotropy, and the anisotropic shift behavior of the resonance field at low temperatures was observed from temperature-dependent ESR, which is attributed to the collective resonance mode associated with anisotropic spin correlations. To probe the single-ion properties of Gd[3+], we performed measurements on magnetically diluted system KBaGd0.005Y0.995(BO3)2, which successfully revealed an easy-plane single-ion anisotropy arising from the crystal electric field effect. The anisotropy parameter for Gd[3+] was determined to be D = 0.053 K, with its energy scale comparable to antiferromagnetic exchange interaction and dipole-dipole interaction. This strongly suggests that the magnetism of in KBaGd(BO3)2 is collectively determined by the interplay of antiferromagnetic exchange, dipole-dipole interaction, and the easy-plane single-ion anisotropy.