Is it Islamically permissible for the investor to receive a fixed monthly amount from an investment, and is zakat obligatory on money whose growth has ceased, on salary, on savings, and on gold designated for marriage?

The questioner did not clarify the conditions and nature of the transaction for a ruling to be made on it, although its apparent form suggests it is a Mudarabah (profit-sharing) agreement. As for Zakat, it is obligatory on saved money if it reaches the Nisab (threshold) and a complete lunar year has passed over it. This also applies to what the questioner saved from his salary. Gold is added to the money and Zakat is paid on the combined amount if a year has passed over it, and one-quarter of one-tenth (2.5%) of the total is to be given as Zakat.