Cannot Speak is a physical, mundane disadvantage, described in Basic Set, p. 125, and referenced in GURPS Space, p. 221, on mitigating the disadvantage with a computer speech synthesizer.

GURPS Horror 78 gives some exceptions:

Even ghosts with Cannot Speak can often utter a single,

programmed phrase (“Get out!” or “Evangeline”), sing a tuneless children’s song, or otherwise unhelpfully allude to their trauma.

GURPS Zombies 69 also:

This is fair for zombies that can vocalize well enough to convey basic information (“Moan once for ‘yes’ or twice for ‘no.’”) – Cannot Speak allows verbal communication, just not spoken language.

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