The troubled midfielder will play in his third league this year after the Japanese side called upon another one of their former players in a bid to stave off relegation
The 26-year-old, who belongs to Spanish outfit Mallorca, joined the Korean side this winter after making just four appearances in the 2011-2012 Liga season. He scored one goal in 12 appearances in this year's K-League.
The announcement marks Ienaga's return to the club he left in 2007. After beginning his career with Gamba, the thrice-capped Japan international was loaned to Oita Trinita and Cerezo Osaka for a total of three seasons before making the jump to the Balearic outfit.
Ienaga is the second former player to re-join Gamba this summer, following the return of Brazilian Leandro after three seasons in Qatar.
The mid-season re-enforcements are expected to help the storied J-League side escape relegation; Gamba currently sit in 17th place, six points from safety.
Ienaga is expected to be eligible to play in the match against Omiya Ardija August 4.