The dreaded Gargoyles of Northrend are voracious flying creatures who revel in slaughter and mayhem. Brought from the frozen north by the armies of the Lich King, these strange, wiry flyers have rough, crystalline hides which protect them from all manner of attacks. In times of great peril, gargoyles can land and condense their hides into a stone-like surface. Though they are unable to attack in this state, Gargoyles can take time to regenerate their wounds and replenish their energies.
Gargoyles are useful for killing other air units and Creeps. If the enemy focuses too heavily on melee units such as Grunts or Huntresses you can use Gargoyles against them. If the enemy builds air units you can use Gargoyles against them if Crypt Fiends are not an option. Generally Gargoyles are used for scaring players with hit-and-run attacks or for dealing with air units. Most Undead players prefer to build up Frost Wyrms instead of Gargoyles.
Use Stone Form to heal the Gargoyles. You can even do it right next to the enemy since the armor is so thick while the Gargoyles are in Stone Form.
Although Gargoyles have the "unarmored" armor type, they still have armor. They have unarmored for balance reasons.