There are three points in the ecliptic at which the boundaries of two zodiac signs or rashis & the boundaries of two constellations or nakshatras coincide. If the moon is close to any of such three points at the time of birth of a person than the person is said to have Gandmool Dosha.
But in actual such a dosha is a myth & has got a lot of exceptions because nothing bad happened to the fathers of such children who saw their faces with in 28 days of their birth, when no pooja was done by any of the pandits.
In actual, relationship with father is observed by analysing tenth house in the birth chart. Such a dosha is not a part of Indian Vedic Astrology & is a mere superstition.