The Honourable Woman star says she discovered the "disappointing" reality when she was turned down for a role because she was "too old to play the lover of a man who was 55," she told entertainment website The Wrap.
"It was astonishing to me," Gyllenhaal said. "It made me feel bad, and then it made me feel angry, and then it made me laugh."
The age of the actor chosen to play the 55-year-old is unknown.
While Gyllenhaal was surprised at being declared undesirable for a man old enough to be her father, the award-winning actress was decidedly more optimistic earlier this year.
While accepting her best actress Golden Globe in January, Gyllenhaal expressed her excitement over the increase in roles for women.
"When I look around the room at the women who are in here and I think about the performances I've watched this year, what I see actually are women who are sometimes powerful, sometimes not, sometimes sexy, sometimes not, sometimes honourable, sometimes not ... that's what I think is revolutionary, evolutionary, and that's what's turning me on," she said.
However, not everyone is thrilled at gender disparity in television and movies.
A 2013 study by the the Center for the Study of Women in Television and Film at San Diego State University found that women represent just 30 per cent of all speaking characters in top movies — the same proportion as American cinema in the 1940s.
A similar study by the same research group found just seven per cent of the top 250 films at the U.S. box office in 2014 were directed by a woman.
As for ageism, boundary pushing comedian Amy Schumer recently skewered Hollywood's complicated relationship with female sexuality in a sketch last month on Comedy Central's Inside Amy Schumer.
Titled "Last F--kable Day," the sketch features Schumer, Tina Fey and Patricia Arquette celebrating Julia Louis-Dreyfus's final day as a "believably" sexually attractive actress.
"No one was more surprised than me that they let me stay f--ckable throughout my 40s," quipped Louis-Dreyfus. "And the fact that it continued into my 50s ... I thought US Weekly had made some kind of a clerical error."