In brucat's example, the SIs would state something to the effect of "all marks left to port, except gates", then the mark would be left to port.

However, if the SIs are silent to that effect - then one could assume that the remaining mark could be either mark of a gate, thus could be rounded in either direction. Obviously, this could be a very bad situation, with boats coming at each other head-on.

That's why if you say you're going to have a gate, then you should have a gate - or a fail safe, like "all marks left to port" in the SIs.