The crystal structure of Gd2B3C2 has been determined from single-crystal X-ray counter data. Gd2B3C2 crystallizes in a unique structure type with the orthorhombic space group Cmmm-D2h19, No. 65, Z = 2. The lattice parameters are a = 0.3445(1), b = 1.3733(3), and c = 0.37107(7) nm. The structure was solved by combined Patterson- difference Fourier methods and refined by full-matrix least-squares calculation. R(F) = SIGMADELTAF/SIGMAF(o) = 0.053 for an asymmetric set of 246 independent reflections (F(o) > 3sigma(F(o))). Boron atoms in triangular metal coordination form infinite zigzag chains branched with carbon atoms. Boron atoms are at a distance of d(B1-B1) = 0.1920 nm with a bond angle of phi(B1-B1-B1) = 127.6-degrees. Branched carbon atoms in 4-fold rectangular metal coordination are at a distance d(B1-C) = 0.1597 nm from the B-B chain under bond angles phi(C-B1-B1) = 116.2-degrees. The boron chains are linked to a planar two-dimensional boron-carbon network by additional boron atoms forming rather tight bonds with the branched carbon atoms at distances of d(B2-C) = 0.1413 nm under bond angles phi(C-B2-C = 180-degrees. With respect to the structural chemical building blocks and near neighbors, the crystal structure of Gd2B2[6Gd+C,2]B[2C,-]C2[4Gd+2B,-] is related to the structure type of YB[6Y+C,2]C[4Y+B,-], which lacks the nonmetal atom linking the carbon-branched boron zigzag chains. The electronic structure and bonding properties of Gd2B3C2 are analyzed by means of extended Huckel tight-binding calculations. The results show that an ionic picture between the metallic and the boron-carbon sublattices is a good starting point to explain the arrangement observed in the nonmetal framework. Thus, a formal charge of 5-per B3C2 unit accounts for the boron-carbon structural arrangement of the B-C net in Gd2B3C2. The electronic structure of the anionic two-dimensional boron-carbon layers is found to be strongly related to that of the boron-carbon layers encountered in ThB2C and alpha-UB2C structures. The possibility of B-B bond alternation is discussed. In the three-dimensional material, the metal-nonmetal bonding occurs primarily through electron donation from the anionic sublattice toward the metallic elements, leading to a metallic behavior.