International cooperation is key to narrow the development gap between nations.

narrow 在这里是动词，意思是"使变窄、缩小"。原形是形容词，表示"狭窄的"，转化为动词后表示"使...变窄"。 narrow the gap 是一个常用搭配，字面意思是"缩小缝隙"，引申为"缩小差距"。