Sentience is self-awareness. Any being with a GURPS IQ of at least 1 is sentient by definition.
Sapience is defined as the ability to use tools and language. In GURPS, this requires at least IQ 6. Those with IQ 5 or less cannot learn technological skills (see p. 168) or possess Languages (see p. 23) – not even the initial Language that most characters get for free. They can still communicate primitive concepts (such as hunger or danger) through gesture or vocalization, and may be trained to respond to a few commands (see Pets and Trained Animals, p. 458).