Accurate automated characterisation of radioactive material is crucial for applications in decommissioning and the detection of illicit material. While detection hardware rarely changes outside of bespoke solutions, improvements in Radio-isotope~IDentification (RID) algorithms have become a significant research focus. A generalised gamma simulator has been built using the GEANT4 toolkit to provide a rapid development environment. With a high-fidelity model using Monte-Carlo based radiation transport, a diverse range of radiation sources and shielding scenarios may be considered. Data derived from facilities such as the Birmingham MC40 cyclotron and Culham Centre for Fusion Energy will provide this diversity for RID performance testing. Experimental validation of the simulator is also performed against real data. An overview of simulation techniques for gamma spectrometry will be presented, along with how the simulator is currently being used to research more robust and reliable RID algorithms.