The idle speed for the series I GS300 is between 650 - 750 rpm.
To test the idle speed ensure the engine is at full working temperature, the car is in neutral and the air con and all electrical devices are switched off. Run the engine at 2,500 rpm for 90 seconds and then allow to idle. Check the idle speed with a tachometer.
The idle speed is controlled by the engine ECU and there is no direct adjustment. If the idle speed is not within specification check all hoses and vacuum lines for any leaks and the air filter is clean. Then clean the throttle body and check the idle air control valve.
Thanks to Colin Barber