What is the song that play while gon use jajanken? What is the song that play while gon uses Jajanken in greed Island arc???
I don't know exactly but it is a track that is played when he uses the full power jajanken against Razor in Soufurabi.. while playing Dodge ball...
The Emperor's Time by 平野義久 , Album : HUNTER×HUNTER セレクトｘベストｘα.

You'll find the details in the description of this video of the battle you mentioned in the comment.