Hi Drive MOSFET waveform for charge controller
The waveform looks like that because it is high side FET gate voltage referenced to ground. So, when the gate is high the peak voltage is Vin + bias voltage, and when gate is low you see whatever voltage is on the switching node (also called PH for some reason on this chip). If the output inductor were in continuous conduction the switching node would be approximately ground. However, since the output inductor is in discontinuous conduction, when gate goes low, switching node goes down to ground, but then the inductor runs out of current and then switching node rings because of the inductance and parasitic capacitance in the circuit. In fact, if you look in the datasheet at Figure 7, you will see just this type of waveform. 

If you wanted to just see the gate waveform, you would need to use 2 probes, 1 for the gate and 1 for the switching node and take the difference (your scope should be able to do that). Then you would see a more square, or really rectangular, waveform.