The crystallization of glass precursors at a moderate temperature of 950°C can be used to isolate highly nonstoichiometric gadolinium aluminate garnets Gd3+xAl5-xO12 with 0 ≤ x ≤ 0.60, which are described here for the first time. Once formed, these materials are highly stable, with composition-dependent thermal decomposition temperatures in the range 1050 -1200°C. Despite the major size contrast between the two cation species, the garnet structure accommodates excess Gd 3+ by replacement of Al 3+ at the octahedral BO6 sublattice: 30% of these sites are occupied by Gd 3+ in the end-member Gd3.6Al4.4O12. The nonstoichiometric range is more extensive than the analogous Y3+xAl5-xO12 system (0 ≤ x ≤ 0.4) due to a broader glass-forming domain in the Gd2O3 -Al2O3 binary system, with the compositional limit of x = 0.6 lying half-way between ideal garnet and perovskite stoichiometries. In common with the nonstoichiometric YAG system, nonstoichiometry has a marked effect on upconversion emission phosphors where dopants can occupy two cation sublattices (AO8 and BO6), demonstrated here in the Tm/Yb-substituted system (Gd0.89Yb0.1Tm0.01)3+xAl5-xO12. This implies that the exploration of other nonstoichiometric RE3+xAl5-xO12 hosts may be particularly interesting for the development of new upconversion systems with small rare-earth dopants, as well as yet-unexplored applications in persistent luminescence and magnetism.