Both Yosuke Ideguchi - after a flowing passing move opened up space for a shot - and Shu Kurata, who volleyed straight at the goalkeeper, were denied early on as Gamba took the game to their visitors.
It took the Purple Archers until the 25th minute to trouble the Gamba defence, when Anderson Lopes latched on to a lucky ricochet, but shot wide from the edge of the area. Just before the break Kohei Shimizu had the chance to blast home a loose ball but was way off target, much to the delight of the home fans behind the goal, who still remembered very clearly the Sanfrecce player's cheating over-reaction antics to get Oh Jaesuk sent off in the J1 Championship play-offs in the final season at Banpaku. 0-0 at the break.
Ademilson and Shun Nagasawa combined on a two-man break just after the restart that forced Hayashi into action down low to his left, but it was the visitors who opened the scoring against the run of play, when a Felipe Silva shot came back off the bar only for Masato Kudo to somehow get his head to it and steer the ball home, also off the woodwork.
Gamba should have been level moments later when Hayashi saw an Endo corner shot straight at him by Genta Miura - just as Nagasawa was dragged to the floor by his marker - and then the desperate defence scrambled away the loose ball. This wasn't the only penalty shout ignored by the referee, as Ademilson had been pulled back by Kazuhiko Chiba in the 1st half, also.
Substitute Ritsu Doan was then denied at the far post by a superb defensive header, and then Hayashi came to his team's rescue again when he clawed away a shot from another Gamba substitute, Jin Izumisawa, in the 81st minute.
Hayashi was then very relieved to see an added time Fabio header come straight at him as Gamba threw everything at the away team. In fact so much were Gamba pushing for a goal that the original cross that led to that header came from Gamba 'keeper Masaaki Higashiguchi, who had had little to do in the match otherwise.
The traveling away fans were obviously ecstatic with the win, however it had come, and Gamba were disappointed they couldn't turn possession and chances into goals, and points. Next up, an Asian Champions League trip - and a must-win match - to Jeju United in Korea, on Tuesday night.
Team - Higashiguchi, Niwa, Fabio, Miura, Oh (Doan, 62), Fujiharu, Ideguchi, Endo (Izumisawa, 67), Kurata, Ademilson (AKasaki, 75), Nagasawa