This technique has never been seen in its normal state but from what can be said from its Fire Monk version (without the flames added), Gabimaru grabs a boulder and hurls it towards his opponent.
Fire Monk Version
Gabimaru can use a Ninpō: Fire Monk version known as Ninpō: Giant Stone — Fire Monk Version (忍法 大礫 火法師拵え Ninpō: Ōtsubute — Hi Bōshi Bājon?). After combining Fire Monk with Giant Stone, Gabimaru sets the boulder on fire and throws it towards his opponent. This is called Ninpo: Grand Crag, Ascetic Blaze Mode in the Viz Manga.