For the EPC device maximum range of Vgs is from -4 V ~ 6 V, is this range a continuous value or transient value?
The VGS range, -4V to +6V, is for both continuous and for voltage transients. When turning off an EPC GaN FET, it is not usual to use a negative voltage (usually 0 V is used for off). While it is possible to use a negative voltage, it is not done because 1) greater gate drive complexity, and 2) not needed for guaranteed turn-off, and 3) negative gate voltage increases the reverse (“diode”) voltage during the dead time which increases losses. So, a little bit of ringing below ground (maybe -0.5 V) is the max. that is typically seen. When turning on an EPC GaN FET, the optimum drive voltage is 5 to 5.25 V. Above that there is very limited benefit. In all, there is from 0.75 to 1.0 V of margin for ringing. If ringing goes above 6V, then it may be good to consider a fix such as a higher series gate resistor or improving the gate drive loop layout.