Gillham performed Connor D'Netto's composition Witness at a Melbourne Symphony Orchestra concert in 2024, introducing the piece with comments about Israel killing journalists in Gaza.
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.
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 Gillham's concert.
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 also 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 testified that he was also surprised and furious that a guest soloist would do such a thing at an MSO concert.
"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.
"Guest soloists are the epitome, the ultimate artistic professionals and the organisation rises to our guest artists, they inspire us."
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.
He recalled the then-chief executive Sophie Galaise telling an MSO leadership meeting that Gillham's comments were highly offensive and would damage the orchestra's reputation.
Also on Tuesday, the trial heard from Morgan Hayes from the Royal Academy of Music, who recalled other political speeches made at classical concerts internationally.
A video Professor Hayes recorded of prominent Israeli conductor Ilan Volkov making an unauthorised onstage speech at the BBC Proms was also played to the court.
"I know that many of us feel completely hopeless in front of this, innocent Palestinians being killed in their thousands," said Volkov.
Giving evidence from the UK, Prof Hayes agreed under cross-examination that guest artists should not offend audiences, or do anything on stage that would harm an orchestra's reputation.
The trial is expected to run for three weeks and the orchestra is expected to call about 20 witnesses.
