In the anime:
Videl finds out when she sees the great saiyaman get a cut from a dinosaur and then sees a cut on the exact spot as Gohan. She puts two and two together and blackmails him by saying that she will keep his secret if he competes in the world Martial Arts tournament and if he teachers her how to fly.
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Gohan teaches her during The Great Saiyaman Saga.
I'm not remember but all I know is Gohan marry Videl after Majin Buu saga
Videl and Gohan got married because they were in love with each other, and as a result wished to be together.
no gohan does not have a son but he does have a daughter named pan and videl is his wife.
they dont