The former captain for the European Ryder Cup team split from Gaynor,50, in 2015 after allegations emerged about the golfer’s private life.
On Tuesday, at the Court of Session judge Lord Doherty heard claims about the alleged unfairness of a “post nuptial agreement” which was signed between the couple in 2010.
The deal, which was signed after Monty,53, was caught cheating with another woman, allegedly committed the sportsman to paying for the upkeep of Gaynor’s £2 million farmhouse.
Montgomerie, who currently lives in Ayrshire, believed the deal wasn’t fair and reasonable.
He is suing Gaynor at the Edinburgh based court for £5 million and is also demanding payments from her of £6,500 per month.
Much of what was said at Tuesday’s hearing cannot be reported for legal reasons.
However, court papers lodged by Mr Montgomerie’s legal team claim that Gaynor placed him under pressure to enter into the “post nuptial” agreement so their marriage could continue.
The papers state that Gaynor, of Dunning Perthshire, had discovered that her husband had been having an extra marital relationship.
Court papers state that Gaynor inherited the farmhouse when her first husband, businessman George Knowles died and it was in her full name before she married Monty.
Her lawyers say the golfer wanted to make changes to the property when he moved in. Gaynor is said to have agreed to then transfer the house into joint names on condition that if they separated he would transfer the title back to her. It is also alleged that he agreed to take all reasonable costs with the house for the remainder of her life.
Gaynor is demanding Montgomerie honours the alleged deal. She is counter suing him for £5 million and £10,000 per month payments.
Montgomerie, who is thought to have a £40 million fortune, captained Europe to Ryder Cup victory in 2010.
He and Gaynor married in 2008.
He and first wife Eimear, the mother of his children, divorced after 14 years in 2004.
He is represented in the current proceedings by advocate Kenneth McBrearty QC. Gaynor’s advocate is Janys Scott QC.
Montgomerie’s solicitors issued a statement to reporters in their client’s name at lunchtime on Tuesday.
In the statement, Montgomerie said: “I will not be commenting on my private life other than to confirm that I raised proceedings some time ago seeking divorce from Gaynor.
“This matter is now being dealt with by the court. I ask that you respect our privacy in this difficult time.
“I will be making no further comment on the matter.”
The hearing before Lord Doherty continues.