Novel Gd-based mixed-metal borohydrides AnGd(BH4)n+3 (A = K, Cs) are described in view of their magnetic cooling properties at cryogenic temperatures. The magnetic entropy changes correlate well with Gd3+ isolation and structural dimensionality in the discovered perovskite-type and other compounds. Crystal structures are resolved from Synchrotron X-ray Powder Diffraction and refined with solid state calculations. The isolated Gd-centres in five-fold coordination in K2Gd(BH4)5, the first representative of a new structure type, lead to exceptionally high mass magnetic entropy change of 54.6 J kg−1 K−1 at the maximum field change of 9 T.