This experience report describes the design and implementation of safety-critical software and hardware for respiratory gating of a medical linear accelerator. Respiratory gating refers to a radiotherapy technique for treating cancer in the lung, liver, and abdomen, where tumors move while a patient breathes. A computer software program tracks the position of the tumor within the human body using x-ray fluoroscopy. When the tumor is in the correct position, the linear accelerator is triggered, delivering a beam of radiation toward the target. As part of the gating system, a comprehensive strategy for safety has been developed. This paper describes these safety features, focusing on the online monitoring techniques used to confirm the proper operation of the fluoroscopic imaging panels and the pattern recognition algorithms used for tumor identification.