Hammered 7-1 by Kashima Antlers in their opening continental hitout, Mark Milligan's side improved against another Japanese foe and even took the lead through Nikos Vergos.
But they couldn't hold their advantage, with Yusei Toshida equalising before halftime and substitutes Deniz Hummet and Welton Felipe striking after the break to send the majority of the 3,219 fans home disappointed.
Defeating Vietnamese side Cong An Hanoi in their opening ACLE fixture but losing four straight domestic fixtures since, Gamba started strongly through Toshida, who had a goal disallowed and forced James Delianov into a save inside 15 minutes.
But as the heavens opened, the Jets dug in and then took a 32nd-minute lead through Vergos.
Collecting the ball at the top of the box, the Greek striker's resulting shot took a deflection off Shota Fukuoka, bouncing upwards and over keeper Jun Ichimori.
This lead only lasted eight minutes, however, as Eli Adams - a favourite son returning to Newcastle after his offseason move to Japan - lofted a cross in for Yoshida to rise and head goalward, catching Delianov out as he scrambled forward.
Daniel Wilmering had an attempt cleared off the line up the other end, but it proved the Jets' last proper sight on goal as Osaka's bench proved the difference. Adams set Hummet up in the 72nd-minute and Felipe bent in an 86th-minute sealer.
The Jets will return to action on Saturday as they open their A-League Men campaign away to F3 Derby foes Central Coast, with concerns for Will Dobson after he was forced off with a shoulder injury.
Their next ACLE fixture will be away to Kyoto Sanga on October 27.
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