The 18-year-old Ramierz got off to a flying start and attacked Nyambayar, 20, from the first bell taking the first round 6-5, but the Mongolian stood his ground in the second which finished 4-all.
Both fighters raised their game in the third with Nyambayar, who beat world champion Aloian in the semi-final, landing some solid shots to the face, but Ramirez came out on top to take the third round 7-5 and secure gold.
His delight was obvious when the result was announced and he danced his way around the ring.