Roper's Gate is an entrance to the city of Vizima that Geralt entered the city through and then headed immediately for the nearest tavern: The Fox. It is one of the northern gates into Vizima, but not otherwise described.
In The Witcher (PC), Vizima is described as having quite a few gates — interior and exterior — but none of them is named the "Roper's Gate". There is a gate shown on the game map of Vizima which is not named and it is the most northerly exterior gate. That could well be it.