Which level 5 monster is the best?

Where did you get this information? Are you saying that if there are 2 Gorgons, there is absolutely no chance of the ability triggering?

Death Gaze really does depend on the number of Gorgons, just like the number of creatures they kill!
For example, take 1 Azure Dragon. How many Gorgons are needed for Death Gaze to trigger? There was a formula somewhere... I forgot.