The regular session closes at one price and reopens at another. Nothing traded in between, and that band of untraded prices behaves like every other imbalance — with a midpoint that matters more than its edges.
Why it exists at all — The regular session has a start and an end, and price keeps moving in between. The difference between the settlement and the reopen is a real, untraded range.
What to mark — High, low and the midpoint. The 50% is where a return most often stalls, and it is the part of the gap most charts never have drawn on them.
What it is not — It is not the same as the overnight gap on a stock. This one is a session boundary inside a market that never really stopped — which is exactly why the untraded band means something.