They never show Gohan and Videl kiss, but at the end of the movie 'Fusion Reborn' it is implied by Goten and Trunks that they saw them kissing when they were spying on them, though it was offscreen.
As for the proposal, it too isn't ever shown. It happens some time after Episode 288 and before Episode 289 (along with the Marriage and Pans birth). Since the movie 'Yo! Son Goku and His Friends Return!!' is in year 776 and Pan is likely conceived in 778 (born in 779) it is to be assumed to have happened at some time in that 3 year span.
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Gohan never admits that he loves Videl, but it is shown when he turns Super Saiyan when witnessing Videl being beaten up by Spopovitch. Videl, though, basically admits it in season 9. Dabura says that he believes he's in the prescence of true love when Videl talked about Gohan
they dont
In Episode:226 Global Announcement when Goku tells everyone that Majin Buu killed Vegeta and Gohan she admits that she loves Gohan.
I'm not remember but all I know is Gohan marry Videl after Majin Buu saga
Gohan and Videl kiss in a movie, not episode. IN FUsion Reborn (Funimation dub only) Trunks and Goten tease them, but they dont show the kiss.