There is no reason to reverse the voltage at 800V.For this device, the average rectified current is 3A volts.There is no maximum value for 125A surge current.Using the output current as an example, 3A is the maximum value.In the datasheets, the peak reverse is 5μA, as indicated by the data chart.
GP30K-E3/54 Features
an average rectified current of 3A volts 3A is the maximum value the peak reverse is 5μA
GP30K-E3/54 Applications
There are a lot of Vishay Semiconductor Diodes Division GP30K-E3/54 applications of single-phase diode rectifier.