here is an updated code for newer GMS2 versions like (v2024.06.2.162)

function Approach(_value,_value_to,_amount)
{

    if (_value < _value_to)
    {
        _value += _amount;

        if (_value > _value_to)
            return _value_to;

    }
    else
    {
        _value -= _amount;
        if (_value < _value_to)
            return _value_to;

    }
    return _value;
}

Keep in mind you can't write the function and expect it to alter a value on its own. You have to write the whole instruction, like this:

y = Approach(y, target_y, move_speed);
and not
Approach(y, target_y, move_speed);