Tactics against Necromancers.

...does the ability of the mighty Gorgons affect Necropolis troops, in particular, Black Knights?

The Deadly Breath of Might and Magic only affects the living. Therefore, Necropolis units are immune to this ability.
And, by the way, on the topic:

- You will also not affect with the breath mummies, gargoyles, golems, and elementals.
- These units are also immune to the cloud of death of liches (if they are standing next to the target).
- "Wave of Death" does not affect only Necropolis units and mummies. "Destroy Undead" - on the contrary.
- Therefore, the spell "Raise Dead" affects not