The Guild, which represents newsroom staffers, advertising-sales representatives and circulation-customer-service representatives, has an existing contract that runs through October.
Contracts for the company's 10 other unions expired last fall.
Gross said that IGM wants the Guild to agree to $8 million in wage-and-benefit cuts and is seeking similar concessions from the other unions.
IGM's ownership group - which includes philanthropist H.F. "Gerry" Lenfest; George Norcross, a South Jersey Democratic power broker and insurance executive, and businessman Lewis Katz - bought the newspapers and their website from a collection of hedge funds in April for $55 million.
The sale marked the fourth time in six years that the media company had changed hands.
Like many other print-media companies, IGM has been plagued by falling circulation and advertising revenue while trying to craft a strategy for getting readers to pay for digital content.
"People felt a little bit of relief when a group of millionaires and/or billionaires bought the company and pledged that they were in it for the long haul," Gross said.
"It surprises me that we've gotten to this point so quickly."
IGM spokesman Mark Block said the company couldn't comment because it was engaged in negotiations with the unions.
On Twitter: @dgambacorta