The Post said Gallagher received a contract from the Department of Health and Human Services in 2002 to promote Mr. Bush's $300 million plan to encourage marriage as a means of strengthening the family.
In a related development, President Bush said he had instructed his cabinet secretaries not to put commentators on the government payroll.
"There needs to be a nice independent relationship between the White House and the press," Mr. Bush said.
The Post said while working for the HHS in 2002, Gallagher wrote in her column that arguments against the president's marriage initiative were "nonsense."
The newspaper also said Gallagher had received an additional $20,000 from the administration for writing a report called "Can Government Strengthen Marriage?"
"Did I violate journalistic ethics by not disclosing it?" Gallagher told the Post. "I don't know. You tell me."
Gallagher later wrote a column in which she said: "I should have disclosed a government contract when I later wrote about the Bush marriage initiative. I would have, if I had remembered it. My apologies to my readers."
Earlier this month, it was learned that conservative Armstrong Williams, a nationally syndicated radio, print and television commentator, was paid $240,000 by the Education Department to promote Mr. Bush's No Child Left Behind Act.
The Post quoted Wade Horn, an HHS official, as saying Gallagher had been hired as "a well-known national expert."
"I don't see any comparison between what has been alleged with Armstrong Williams and what we did with Maggie Gallagher," Horn told the Post. "We didn't pay her to write columns. We didn't pay her to promote the president's healthy marriage initiative at all. What we wanted to do was use her expertise."