In this study, we investigated the glass-forming ability (GFA) and magnetocaloric effect (MCE) of $Gd_{55}Co_{20}Al_{23}Si_2$ bulk glassy alloy. It is found that the addition of 2 at% Si is effective for extension of the supercooled liquid region (${Delta}T_x$), the ${Delta}T_x$ is 55 K for the $Gd_{55}Co_{20}Al_{25}$ glassy alloy, and increases to 79 K for the $Gd_{55}Co_{20}Al_{23}Si_2$ alloy. As a result, $Gd_{55}Co_{20}Al_{23}Si_2$ glassy alloys with diameters up to 5 mm were successfully synthesized. The alloys also exhibit large MCE, i.e., the magnetic entropy change (${Delta}S_m$) of 8.9 J $kg^{-1};K^{-1}$, the full width at half maximum of the ${Delta}S_m$ (${delta}T_{FWHM}$) of 87 K, and the refrigerant capacity (RC) of 774 J $kg^{-1}$.