The gallery normally opens on Mondays, so Leo is surprised to find the door locked. Complete his question: "___ open today?"

Erklärung

The speaker expected the gallery to be open, so the negative question "Isn't the gallery open today?" naturally signals his surprise. Copular be inverts directly with the subject; forms such as "Does the gallery not be" and "Doesn't the gallery be" wrongly add do-support.