The engines windmill and during that windmill actually still generate hydraulic power, which could be enough to extend the gear, but it is not nearly guaranteed to work. Dunno about other airlines, we have an Emergency Landing (all engine failure) checklist on the back of our normal checklist, which does call for gravity extension of the landing gear. That checklist is meant for the first you cite, complete loss of thrust close to the ground. I'm pretty sure it is a company procedure, not an airbus one, we did not have that in a previous airline i flew for that was on 100% airbus procedures.