It's amazing how many people on here don't know the rules.
Before var can be checked, the ball must go out of play. Gordon was offside which led to there being a stoppage in play and then the incident can be checked.
The man on VAR checks to see if there was a clear and obvious error. If he believes there was, he then checks the build up to the goal and see if there was anything that occurred that means the penalty can't be given.
As he deemed it wasn't a clear and obvious error, there was no need to check if Richarlison was offside.
He then indicates to the ref that the game can continue from where it stopped, the Gordon offside.
Technology was never used to see if Richarlison was offside. Anyone who says he was offside from a dodgy angled screenshot is stupid.