Background: In respiratory gated radiotherapy, low latency between target motion into and out of the gating window and actual beam-on and beam-off is crucial for the treatment accuracy. However, there is presently a lack of guidelines and accurate methods for gating latency measurements. Purpose: To develop a simple and reliable method for gating latency measurements that work across different radiotherapy platforms. Methods: Gating latencies were measured at a Varian ProBeam (protons, RPM gating system) and TrueBeam (photons, TrueBeam gating system) accelerator. A motion-stage performed 1 cm vertical sinusoidal motion of a marker block that was optically tracked by the gating system. An amplitude gating window was set to cover the posterior half of the motion (0–0.5 cm). Gated beams were delivered to a 5 mm cubic scintillating ZnSe:O crystal that emitted visible light when irradiated, thereby directly showing when the beam was on. During gated beam delivery, a video camera acquired images at 120 Hz of the moving marker block and light-emitting crystal. After treatment, the block position and crystal light intensity were determined in all video frames. Two methods were used to determine the gate-on (τ_on) and gate-off (τ_off) latencies. By method 1, the video was synchronized with gating log files by temporal alignment of the same block motion recorded in both the video and the log files. τ_on was defined as the time from the block entered the gating window (from gating log files) to the actual beam-on as detected by the crystal light. Similarly, τ_off was the time from the block exited the gating window to beam-off. By method 2, τ_on and τ_off were found from the videos alone using motion of different sine periods (1–10 s). In each video, a sinusoidal fit of the block motion provided the times T_min of the lowest block position. The mid-time, T_mid-light, of each beam-on period was determined as the time halfway between crystal light signal start and end. It can be shown that the directly measurable quantity T_mid-light − T_min = (τ_off+τ_on)/2, which provided the sum (τ_off+τ_on) of the two latencies. It can also be shown that the beam-on (i.e., crystal light) duration ΔT_light increases linearly with the sine period and depends on τ_off − τ_on: ΔT_light = constant•period+(τ_off − τ_on). Hence, a linear fit of ΔT_light as a function of the period provided the difference of the two latencies. From the sum (τ_off+τ_on) and difference (τ_off − τ_on), the individual latencies were determined. Results: Method 1 resulted in mean (±SD) latencies of τ_on = 255 ± 33 ms, τ_off = 82 ± 15 ms for the ProBeam and τ_on = 84 ± 13 ms, τ_off = 44 ± 11 ms for the TrueBeam. Method 2 resulted in latencies of τ_on = 255 ± 23 ms, τ_off = 95 ± 23 ms for the ProBeam and τ_on = 83 ± 8 ms, τ_off = 46 ± 8 ms for the TrueBeam. Hence, the mean latencies determined by the two methods agreed within 13 ms for the ProBeam and within 2 ms for the TrueBeam. Conclusions: A novel, simple and low-cost method for gating latency measurements that work across different radiotherapy platforms was demonstrated. Only the TrueBeam fully fulfilled the AAPM TG-142 recommendation of maximum 100 ms latencies.