The mother-of-one was holidaying in Iowa with her family when the accident happened, reports said.
According to The End Run, the 46-year-old had gone swimming in West Lake Okoboji on the evening of August 6 but did not return.
Her body was found by a family member the next morning.
Irish-born Gallagher worked at the University of Colorado Hospital, where many of the victims of the shooting were taken in the aftermath of the theater massacre.
Twelve people were killed and 58 injured at a midnight premiere of "The Dark Knight Rises" at Century theater in Aurora, Colorado, in July.
"She worked the morning after the massacre in a very busy unit of the hospital — so she saw everything really, some really bad injuries,” her husband, Greg Pinson, reportedly told Ireland’s Herald.
CBS News reported she cared for some of the most critically injured patients in the neurosurgical intensive care unit.
"She did bring home a lot of the tragedy and talked about it," said Pinson.
Gallagher was among the medical staff who met and were praised by President Barack Obama on July 22 for saving the lives of many wounded in the random shootings, the End Run reported.
A photograph of Gallagher and her collegues with Obama was taken on the day.
Pinson said he was heartbroken at the tragedy, the New York Daily News reported.
"She wasn't a very strong swimmer and I suppose she just wasn't able to stay afloat,” Pinson.