After India had been set a daunting target of 444 by Australia, Cameron Green took a stunner at gully to dismiss Gill for 18 even though opinions differed on whether it was a clean catch.
Video replays could not clearly establish if the ball had brushed the ground when Green caught it and third umpire Richard Kettleborough ruled in Australia's favour.
Gill subsequently tweeted a front-on angle of the catch adding two magnifying glasses and a facepalm emoji to it to convey his disbelief.
The 23-year-old's post breached article 2.7 of the ICC's code of conduct relating to "public criticism of, or inappropriate comment in relation to an incident occurring in an international match", the ICC said in a statement.
Gill accepted the sanction so there was no need for a formal hearing, it said.
India captain Rohit Sharma called the decision "a little disappointing", and said more camera angles could have removed any doubt.
"There was only one or two camera angles that were shown," Rohit said after their comprehensive defeat and compared it with the Indian Premier League (IPL).
"We've got 10 different angles in IPL. I don't know why in a world event like this, there was no ultra motion that was seen or any kind of zoomed image was seen."
India were also fined their entire match fee, while Australia were docked 80% of their fees for maintaining slow over-rates at The Oval.
India were found five overs short of their target, while Australia were four overs behind.
"It is disappointing when you don't get as many overs in as you would like," Australia captain Pat Cummins said after their victory.
"The good thing is we still have a result."