McGrath, 20, signed a formal release from his contract this week with Brisbane, citing injury issues and a loss of motivation as factors behind his choice.
"It's a difficult choice to make but I want to be honest with myself and with the club – and I just don't have the desire to continue at the elite level," McGrath said in a statement.
"Football is something I love and I want to get back to a place where I can enjoy the game again.
"I'd like to thank my family and friends for all their support, and the club for giving me an opportunity to have a crack at AFL footy."