The 24-year-old, who has a £3 million release clause, is being linked with a move to Premier League newcomers Cardiff City, according to The Scottish Sun.
Windass bagged a brilliant 18 goals in all competitions for Rangers last season.
However, the former Accrington Stanley playmaker has never been unanimously adored by fans of the Ibrox club.
Gerrard is slowly but surely building a revolution in Glasgow, with the legendary midfielder signing up seven players already - six on permanent deals, and Ovie Ejaria on loan from Liverpool.
And Johnstone reckons that Windass won't be part of the 38-year-old's project with the Gers for long.
"If someone has some doubts about whether they want to be a part of it, then they know where the door is," he told The Glasgow Evening Times. "This is why I believe Josh Windass will leave if Cardiff City or another club bid £3m for a midfielder.
"Even if he is 50/50 about making a move, he goes. Rangers is not for him and he would leave with my best wishes."
He might not be the most popular Rangers player, but 18 goals speaks for itself and if Windass goes then it is hugely important that Gerrard signs a grade-a replacement.