Elongated micro- and nanostructures of Sn doped or Sn and Cr co-doped monoclinic gallium oxide have been grown by a thermal method. The presence of Sn during growth has been shown to strongly influence the morphology of the resulting structures, including Sn doped branched wires, whips, and needles. Subsequent co-doping with Cr is achieved through thermal diffusion for photonic purposes. The formation mechanism of the branched structures has been studied by transmission electron microscopy (TEM). Epitaxial growth has been demonstrated in some cases, revealed by a very high quality interface between the central rod and the branches of the structures, while in other cases, formation of extended defects such as twins has been observed in the interface region. Cathodoluminescence (CL) measurements show a Sn-related complex band in the Sn-doped structures. In the Sn−Cr co-doped samples, the characteristic, very intense Cr3+ red luminescence emission quenches the bands observed in the Sn doped samples. Branched, Sn−Cr co-doped structures were studied with microphotoluminescence imaging and spectroscopy, and waveguiding behavior was observed along the trunks and branches of these structures.