We have developed , a modular, flexible, and efficient end-to-end simulator for submillimetre (submm) integral field units (IFUs) that use kinetic inductance detectors (KIDs). The simulator consists of a Python interface, powered by a C/C++ backend that uses CUDA for GPU-acceleration, and is publicly available and fully open-source. gateau gateau simulates observations by taking user input such as an astronomical source, a set of atmospheric screens, a scan pattern, and telescope and instrument parameters. The source signal is propagated through a dynamical model of the atmosphere and optical path at the telescope using a customisable radiative transfer cascade. A dispersive element model, which can be a filterbank, grating, or user-supplied model, is used to calculate the total power per spectral channel, for each spatial pixel on the IFU. A physically motivated photon-noise model is used to add a white noise component to the received power. Detector noise is added as temporally correlated pink noise. The output is stored in the form of time-ordered datasets. We validated against observations with DESHIMA 2.0, a superconducting, ultra-wideband spectrometer utilising KIDs and on-chip filterbank technology. We show that we can reproduce real observations of the atmosphere and Uranus with simulations. Then, we present an example use case to show how can be used to simulate long observations of a faint source. Finally, we investigate the scalability of and argue that it can be used to simulate large IFUs with detector counts of up to 10^7 KIDs. gateau gateau gateau gateau We conclude that is a suitable instrument simulator for large submm IFUs using KIDs and that it will prove crucial for the design, testing, and operation of these instruments, along with observation plans, calibration strategies, and data reduction pipelines. gateau