Here, we focus on RE = Gd 3+ (GAG), which has been investigated extensively for its optical properties including luminescence and scintillation, and for its complex low temperature magnetism. Gd3Al5O12 lies close to the stability limit of the RE3Al5O12 series (RE = Eu -Lu, Y) and is itself metastable, but it is readily synthesised in polycrystalline form by sol-gel and glass crystallisation methods. Polycrystalline GAG-based phosphors are more easily prepared when the garnet phase is stabilised by substituents that reduce the A/(B,C) ionic radius ratio (e.g. Lu 3+ for Gd 3+ ; Ga 3+ for Al 3+ ). The substitution of Gd 3+ by larger RE 3+ ions increases this ratio and rapidly destabilises the garnet structure: Eu3Al5O12 is significantly more challenging to synthesise, and to the best of our knowledge there are no reports of bulk Sm3Al5O12. As Gd 3+ is the largest RE 3+ ion to yield a readily-synthesisable RE3Al5O12 garnet, compared to YAG, it may be possible to distribute a wider range of RE 3+ dopants over multiple sublattices due to the larger B site environment induced by nonstoichiometry in a Gd3+xAl5-xO12 system. Here, we explore the compositional limits of the highly nonstoichiometric GAG system Gd3+xAl5-xO12 synthesised by glass crystallization, and investigate experimentally the structural and luminescence responses of three different phosphor types (upconversion and broad-/narrow-band photoluminescence) to nonstoichiometry.