Have you tried printing the angle from the returnPIDInput() method?
Could you be turning the opposite direction that you want to be? If you turn right and that causes the angle to count up you'll never get to -90. You'll probably also want to set your setpoint to something like
Code:
Robot.driveTrain.setSetpoint(-90.0 + Robot.driveTrain.getGyroAngle());
That way it'll always turn 90 degrees from your currently angle.