What do we mean by "gate plateau voltage"?
It's called a plateau because it's relatively flat, compared to the steep line before and after it.

It's caused by the Miller effect of the drain-gate capacitance holding the gate voltage relatively constant, despite charge being supplied to the gate by the gate driver.

In the first steeply rising below gate threshold region, the gate current is only charging the small inter-electrode capacitances.

At gate threshold, the Miller effect multiplies the DG capacitance by several orders of magnitude, reducing the rate of rise.

Once above the gate threshhold, the capacitance reverts to the inter-electrode capacitances, and the voltage rises steeply again.