This is the first of three papers describing an ‘absolute’ calibration of the GONG magnetograph using an end-to-end simulation of its measurement process. The input to this simulation is a MURaM 3D MHD photospheric simulation and the output is the corresponding set of simulated data numbers which would be recorded by the GONG detectors. These simulated data numbers are then used to produce ‘synthetic magnetograms’ which can be compared with the simulation inputs. This paper describes the GONG instrument, the MURaM datacube, our instrument simulator, and calculation of synthetic magnetograms, setting the stage for the subsequent two papers. These will first lay groundwork for calibration (and magnetogram comparison in general), then apply them to calibration of GONG using the simulation results.