Low-dimensional lead-free metal halides have emerged as novel luminous materials for solid-state lighting, remote thermal imaging, X-ray scintillation, and anticounterfeiting labeling applications. However, the influence of band structure on the intriguing optical property has rarely been explored, especially for low-dimensional hybrid heterometallic halides. In this study, we have developed a lead-free zero-dimensional gallium-bismuth hybrid heterometallic halide, A8(GaCl4)4(BiCl6)4 (A = C8H22N2), that is photoluminescence (PL)-inert because of its indirect-band-gap character. Upon rational composition engineering, parity-forbidden transitions associated with the indirect band gap have been broken by replacing partial Ga3+ with Sb3+, which contains an active outer-shell 5s2 lone pair, resulting in a transition from an indirect to a direct band gap. As a result, broadband yellow PL centered at 580 nm with a large Stokes shift over 200 nm is recorded. Such an emission is attributed to the radiative recombination of an allowed direct transition from triplet 3P1 states of Sb3+ based on experimental characterizations and theoretical calculations. This study provides not only important insights into the effect of the band structure on the photophysical properties but a guidance for the design of new hybrid heterometallic halides for optoelectronic applications.