Magnetic cooling technology based on magnetocaloric effect (MCE) has attracted great interest in obtaining extreme low temperature, e. g. for space exploration. Here we grew a new gadolinium-rich orthoborate K3Li3Gd7(BO3)9 (1) as a promising cryogenic magnetic coolant. It exhibits complicated three dimensional framework constructed from BO3 groups and gadolinium-oxygen chains. The Gd-O chain consists of two types of clusters of Gd3O20 and Gd3O19 interconncetion by Gd(4)O8 polyhydron. Thanks to its high gadolinium concentration, a large -Sm for 1 was obtained as 56.6 J kg-1 K-1 at 2 K and H = 7 T, much larger than that of commercial benchmark Gd3Ga5O12 (GGG) crystal (38.4 J kg-1 K-1), suggesting it to be an excellent MCE material.