In this study, we investigated the glass-forming ability (GFA) and magnetocaloric effect (MCE) of Gd₅₅Co₂₀Al₂₃Si₂ bulk glassy alloy. It is found that the addition of 2 at% Si is effective for extension of the supercooled liquid region (ΔT_x), the ΔT_x is 55 K for the Gd₅₅Co₂₀Al₂₅ glassy alloy, and increases to 79 K for the Gd₅₅Co₂₀Al₂₃Si₂ alloy. As a result, Gd₅₅Co₂₀Al₂₃Si₂ glassy alloys with diameters up to 5 mm were successfully synthesized. The alloys also exhibit large MCE, i.e., the magnetic entropy change (ΔS_m) of 8.9 J kg⁻¹ K⁻¹, the full width at half maximum of the ΔS_m (δT_FWHM) of 87 K, and the refrigerant capacity (RC) of 774 J kg⁻¹.