Apparently the sea level difference between the Mediterranean and the Red Sea (actually the Gulf of Aqaba) was measured in 1963 as 24cm, which wouldn't be enough to require a lock once the losses due to evaporation and the channel resistance are taken into account. Certainly the Egyptians couldn't have measured this, the flying saucers had long departed, so I suspect priestly resistance at work here. Or perhaps the gate was more to protect the canal from Nile floods.