External door & ramp
The door to the ship is a vertical slit in the otherwise seamless fuselage. The ramp extends and retracts as part of the door’s function. Gort is the only one in control of it. Klaatu can instruct Gort to open it with the command “Beringa!” and Gort wirelessly initiates the opening sequence.
This might seem to be a questionable security feature, since if Gort is not around, Klaatu cannot enter into the sanctuary of his ship. Fortunately Gort is both indestructible and immovable. The only time he leaves the proximity of the ship is after Klaatu dies. So it seems like his is like an invincible, semi-intelligent, and loyal concierge.
The internal doors are operated by proximity, but only open for Gort and Klaatu. They remain shut even as Helen pounds against them. This implies a combination of proximity sensor and some form of authentication. Since the ship does not display any of the intelligence that Gort does, this authentication is more likely to be something like an RFID chip than biometric authentication.