Gravity. From one operators training notes
The landing gear is hydraulically actuated and mechanically controlled. The only electrical items related to the gear are the gear indication, warning, squat switch functions, and anti-skid. With loss of electrical power, the gear may be extended normally, but will not retract because of the down lock solenoid. If you are unfortunate enough to have a hydraulic failure, the gear may free fall to the extended position if the fluid trapped in the uplocks leaks out. Prior to a long over water flight, some operators check the uplocks by jacking the airplane, retracting the gear, bleeding off the hydraulic pressure, and letting the aircraft sit for two or three hours to see if the uplocks hold. Israeli Aircraft makes fine airplanes, however, there are a few things that indicate some level of brain damage in one or more members of their design team. This is one of them. The range and speed of the airplane is somewhat more limited with the gear down.
The landing gear is hydraulic. What a surprise! It is powered by the main hydraulic system. The normal extension and retraction of the gear is not dependent upon electrical power, as in many aircraft. The gear handle on the Westwind actually moves a hydraulic valve that operates the gear. Indication and anti-skid protection is all that is lost if an electrical failure occurs after the gear is retracted. If you have an electrical failure prior to retracting the gear, the ground safety solenoid will not let you move the gear handle out of the down position. It does take electrical power to override this. The most likely use for the override would be the failure of a squat switch followed by an engine failure after V1, but prior to gear retraction, as leaving the gear down would result in a substantial loss of climb performance.
Alternate extension of the gear is done by placing the gear handle in the down position, unlatching the emergency gear handle and rotating it 90 degrees aft, and pulling it up to discharge the nitrogen into the down side of the forward main landing gear actuators. The nose wheel is extended by a bungee.
If the main hydraulic system is not providing pressure to the brakes, the brake pedals can be depressed farther by the pilot's feet. This activates the emergency braking system. The emergency brake system takes fluid from a standpipe in the main hydraulic system reservoir. An electric hydraulic pump supplies brake pressure to the aft brake calipers only. There is no anti skid protection when using the emergency brake system. The emergency brake system is what you use to set the parking brake prior to starting engines, as you most likely will not have main system hydraulic pressure until one of your engines is started.