An Apex gang member who pointed a sawn-off shotgun in a man's face during a carjacking must spend 20 months in youth detention.

Isaac Gatkuoth, now 19, was affected by the drug ice and wearing a mask with a smiley face on it when he got out of a white BMW - that was later involved in a fatal crash - and pointed the shotgun at the man and demanded his car keys in Frankston South in the early hours of November 5 last year.

Four days later in Mordialloc, the BMW collided with a car driven by Amanda Matheson, a 47-year-old mother of two, who suffered serious injuries and died in hospital.

A teenage boy is to appear before a children's court charged over the fatal crash.