Herein, the influence of the oxygen flow on the formation of metastable polymorphs of gallium oxide (Ga₂O₃) grown by halide vapor phase epitaxy (HVPE) on c-plane patterned sapphire substrates (PSS), on gallium nitride (GaN) templates, and on m-plane smooth sapphire substrates is reported. X-ray diffraction, scanning electron microscopy, and cathodoluminescence are used to identify different polymorphic phases. The samples deposited on bare PSS exhibit faceted growth of the α-Ga₂O₃ on the cones of the sapphire substrate and the formation of the κ-Ga₂O₃ between the cones. In contrast, growth on GaN templates results in hexagonal columns of κ-Ga₂O₃ which produce a continuous smooth layer upon coalescence. The growth of Ga₂O₃ on m-plane sapphire substrates results in overgrown pyramids of the α-phase. For all types of substrates, the variation of the oxygen flow affects only the thickness and coalescence of Ga₂O₃ layers. Thus, the growth of Ga₂O₃ metastable polymorphs is mainly influenced by the form, orientation, and symmetry of the substrate and shows a weak dependence on stoichiometry.