Originally Posted by
Santander
The actual term is gear door, so you're correct.
The manual extension should release the door and let the gear "fall down". Some aircraft do not have gear doors at all. In the 767's case, if there was some sort of electrical issues it is definitely possible the door didn't open.
According to reports the issue was hydraulic. One other point to make is that whilst there may have been a manual system to release the gear and mechanically lower it the other issue is ensuring that it locks into place. The issue then is to land without the gear and land with a troublesome gear and risk a collapsing landing gear which may have been more dangerous.