Gable was laid to rest in a marble tomb beside his third wife, actress Carole Lombard, at the memorial park's Great Mausoleum. He died after suffering a heart attack in November 1960.
According to the late icon's son, John Clark Gable, lipstick smears have frequently appeared on his father's tomb in the past, but have been easily removed.
However, John was forced to call in Forest Lawn's damage repair team after discovering a permanent stain during his visit to the mausoleum last month (May10).
He tells TMZ.com he worries other celebrities buried at the site, including pop icon Michael Jackson, may also face vandalism despite prior security measures to ban overzealous fans.
John reveals Forest Lawn security guards have been asked to "keep an eye out" for potential vandals.