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. 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.