A means for electrically gating a microchannel-plate (MCP) detector using a fast rise-time, x-ray-driven Auston switch is presented. The Ga:As Auston switches which are used in the pulser were tested to stand off 7 kV DC bias, and can therefore be used without ramped biasing electronics. Calculations for the x-ray sensitivity of the Auston switches in the pulser are presented. The pulsers were tested with visible light and x rays and were shown to exhibit 90 ps rise-time between zero and 1 kV when triggered by a 1.2 mJ/cm2 flux of subkilovolt x rays as measured by silicon p-i-n diodes. The pulser was designed to trigger MCP detectors which were used to spectrally characterize an x-ray heated target. A major attribute of this pulser is that it can be preset to gate a detector with less than a few picoseconds timing jitter with no foreknowledge of the time of arrival of the triggering x-ray flux.