From the B767 FCOM:
The alternate landing gear extension system uses an electric motor to trip the locking mechanism for each gear.
No electrical power, no gear extension, regardless of status of the center hydraulic system. This is the precise result when the CB controlling the motors is popped.
Can't seem to find it, but recall from a previous aircraft's QRH a warning to extend landing gear prior to battery depletion in a "Loss of All Generators" c/l.