Board Thread:Questions and Answers/@comment-10797017-20130622101437/@comment-11051098-20130708211405

If Isshin never used GT while being a captain, no one could possibly question it as no one knew that the technique had been used by 2 persons. Also, no matter if it should be a Shiba technique, that other Shiba clan members could also use, it's not certain that Rukia ever saw Kaien use it.