could you please explain why my FET gate will be ruined from fast pulse?
Because with such fast edges your blocking inductor and gate capacitance may ends up causing large amplitude ringing, and GaN FET gates are usually much more fragile than other types of FETs.
Anyway,
You may get much better advice here if you just tell us what you're trying to do, or at least letting us know a bit more instead of asking many piecemeal question on very specific functions that may or may not even relevant to what you're doing.
From your other questions I was assuming you're trying to make a high power RF amplifier or some sort. But frankly the details aren't quite add up, part choices are a bit weird, and feels like you *may* misunderstood some basic engineering detail that *may* lead you to some weird parameter choices.