Why do I need to use a pulse generator capable of 5 volts into 50 Ω?

The benefit of using a gate input with an impedance of 50 Ohms is that a low-input impedance reduces false triggering from noise.

If you’d like to convert the PCO-7125 gate input to high-impedance, you can remove R15 and R19 (100Ω resistors in parallel). However, this configuration may result in unpredictable pulse output.