As a practical matter when sailing, I would round the existing mark as if there were no gate.

As a technical matter, things would be more complicated. Did the sailing instructions specify a leeward gate, or did they allow a single rounding mark? If they conflict with the Notice of Race, then RRS 63.7 comes into play, and the protest committee needs to decide which one will apply.

If the Race Committee set a gate, and one of the marks is missing, then RRS 34 applies. The RC shall, if possible, either replace the mark, or substitute an object displaying flag M. Failing to set a required mark, or failing to replace one that is missing is an improper omission on the part of RC and may be grounds for redress (see RRS 62.1(a)).

Things could get really complicated if boats chose to round the mark in opposite directions. As a Protest Committee member, I'd be inclined to accept roundings in either direction. As a competitor, I wouldn't count on PC sharing that interpretation.

I hope that helps,
Eric