what is causing this ring effect on my MOSFET testing circuit and how would I be able to calculate the MOSFET power losses?
The signal rise/fall times are so fast that either you have incorrect scope probing method with long ground clip, or there is too much stray inductance from the PCB wiring. The series resistance slows down signal edges as it forms a RC filter with the FET gate capacitance.

If the probing is correct and the effect is real, then you need a snubber network to dampen the ringing.