User blog comment:RoyalGuard-Elite/Isshin's Shikai/@comment-4509522-20131014173201/@comment-4207271-20131016212742

Getsuga always struck me as a technique, not an ability, however it would not be unheard of for a Zanpakuto user to use a Shikai ability in sealed state (Rose using Arpeggio in sealed state) We were never told whether Getsuga was actually a Zanpakuto ability (only told be Ichigo, who was told by Qangetsu) I guess, only Kubo will tell.