The 22-year-old is said to have died on Sunday. He used to play for the Chris Gayle Academy in London.
Adrian was attacked after he decided to stop his car to pick up friends. He was shot as he tried to drive away after being robbed, reported BBC on Tuesday.
West Indies cricketer Gayle tweeted on Monday: "Such sad news to hear. My condolences to his family & friends. Adrian St John was the captain of the academy. #Sigh."
Donovan Miller, programme manager for the Chris Gayle Foundation, was quoted as saying by the BBC on Monday: "I'm really finding it hard to come to terms with it, how someone could do something like that to such a lovely person."
"He wanted me to support him to go to the Caribbean and to kind of push hard with his cricket."