Which gate mark?
The situation
The leeward mark is a gate (two marks). Coming down overlapped on the inside, Owen is committed to rounding the left-hand gate mark; the outside boat is set up for the same one.
The question
How does mark-room work at a gate, and what does Owen get?
The ruling
A gate is just two marks; rule 18 applies to whichever one you're rounding. For the left-hand mark, the boats are treated as at that single mark: if Owen is overlapped inside when the first reaches that mark's zone, the outside boat owes him mark-room (rule 18.2). Crucially, mark-room is decided separately for each gate mark — a boat heading for the other gate mark is in a different rule-18 situation. Pick your gate early so you know which mark's zone and overlap count.
At a gate, rule 18 works mark-by-mark. Commit to a gate mark early — your mark-room is judged at that mark's zone.
Opens the situation on the boat-length grid — scrub it and see exactly how the boats meet. Free, no account needed.
Rules cited
See the underlying rule: Rule 18 — Mark-Room.