In the ERA report, she said Mr Petronelli told her he would not keep her job open or pay her maternity leave - claims he disputes.
Later that month she claimed a row erupted between the pair after Petronelli accused Ms Gibson of undercharging two clients.
He became "very aggressive", yelled at her and said she had been sending clients to a salon around the corner, she said in the report.
She told her boss she had previously sent a client to another salon because the woman had wanted a colour they did not have, so they couldn't help her, and that she had earlier told her boss about it.
The disagreement escalated with Mr Petronelli calling her "selfish and ungrateful" and saying she did not deserve her wages, she said in the report.
Gibson said she was then forced to write a letter of resignation and left in tears.
Mr Petronelli today (Tuesday) refuted her version of events. He said the pair had previously been on good terms until the disagreement, after he learnt from a source she was sending his clients to other salons.
"I asked if she was sending clients elsewhere and she said 'no'. I said fine," he said.
He told Ms Gibson he would be taking away her previous privileges such as leaving early if there was no work and having time off for a scan.
Ms Gibson then told him she would resign and he asked her to put it in writing, he said.
"The pregnancy was never any problem. It was never an issue."
The ERA found Ms Gibson was constructively dismissed and Mr Petronelli's behaviour was in breach of the employment agreement.
Her pregnancy was not the reason for the dismissal, ERA member Dzintra King found.
He was ordered to pay compensation of $4,000 to Ms Gibson for humiliation and distress.
She was also entitled to 14 weeks' maternity leave payments, a maximum of $458.82 before tax each week, which she would have received if she had not been dismissed, the ERA found.
Mr Petronelli said he would be appealing the decision: "I'm not paying a cent. I've done absolutely nothing wrong," he said.
Ms Gibson declined to comment on the decision.
Labour's spokeswoman for Women's Affairs, Sue Moroney, said there was a concerning increase in the number of complaints of discrimination against pregnant women.
The Human Rights Commission has dealt with 84 complaints in the last two years, and the Whanganui case "highlights an unhealthy change in attitudes towards the value of family life," she said.
"The Government should be challenging these attitudes, said Ms Moroney.
