In mythology, the Gae Bolg is super deadly not because of its quality as a weapon, but because it could kill from a single scratch. After piercing the opponent, the Gae Bolg would spread no less than thirty barbs through its victim, killing them instantly. (Could also only be thrown with the toes, but I digress.)
Since Ferdiad's hide was impenetrable, Cu Chulainn instead gave his foster brother an unwanted rectoscopy - he shoved the Gae Bolg up Ferdiad's ass, triggering its enchantment and killing him.