Gillham performed Connor D'Netto's composition Witness at a Melbourne Symphony Orchestra concert in 2024, introducing the piece by saying Israel had deliberately targeted journalists to prevent the reporting of war crimes.
The orchestra responded by cancelling his next appearance and the performer is suing the MSO for unfair dismissal in the Federal Court.
The case is expected to test the limits of political speech for contractors in Australian workplaces.
When MSO executives rang Gillham's agent Elaine Armstrong to cancel his next engagement, she said her client's behaviour had been selfish, the court was told.
Ms Armstrong said her client's onstage commentary was a complete nightmare, according to notes of the call taken by MSO executive Guy Ross.
"(It was a) middle finger to those who have helped him build up his career, the concert stage is not a place for that," she said, according to his record of the conversation.
Mr Ross, who is a named respondent in the court case, testified he was surprised and furious a guest soloist would do such a thing at a MSO concert.
Gillham's actions were a breach of trust that demonstrated disrespect and a complete disregard, he told the court.
"Crazy situation from Jayson, bloody hell, I'm furious," he texted a colleague after finding out about Gillham's comments.
"I expected something like this, but not from a soloist," Mr Ross testified.
He gave evidence that all aspects of MSO guest soloist performances were worked out in advance, including any intentions to express an opinion on stage.
The concert had a "senior audience" that included a lot of Jewish people who would have been upset, Mr Ross said.Â
On the second day of a 15-day trial, the court heard details of senior orchestra management's handling of the crisis in the days after the concert.
He recalled the then-chief executive Sophie Galaise telling an MSO leadership meeting Gillham's comments were highly offensive and would damage the orchestra's reputation.
In text messages with another colleague Mr Ross suggested terminating Gillham's contract, an approach later adopted by the meeting.
Mr Ross testified he would still have suggested termination if the pianist had expressed an onstage opinion about Israeli hostages, or Aboriginal disadvantage.
"It was done without any care or consent or engagement, it would have had the same effect," he said.
The MSO argued Gillham should have informed the orchestra of his intentions to express political opinions onstage.
But under cross-examination Mr Ross testified the MSO would have objected, had it known precisely what the pianist was going to say.
Gillham gave evidence under cross-examination on Monday that he had not told his agent or the MSO he would be making political comments at the concert.
