Elsewhere at the Nagoya-based Games on Saturday, Eom Ji-sung scored the winner in the men's soccer final at Toyota Stadium as South Korea held off Japan 1-0 in another final to extend their dynasty to a fourth successive men's gold.
Two years before cricket returns to the Olympics, India cruised to a 19-run win over Pakistan, with Abhishek Sharma and Tilak Varma smashing fifties in the T20 final.
"A huge occasion when you are playing Pakistan in the final, the whole country is watching and you have that pressure," said India captain Shreyas Iyer.
A jet-lagged Kim felt the pressure on the final day of the golf at the Kasugai Country Club, two years after narrowly missing out on a medal at the Paris Olympics.
The four-times US PGA Tour winner played superbly, though, carding a final-round 64 to finish three strokes clear.
South Korea offers Asian Games champions an exemption from compulsory military service.
"This day was the longest day I think I've ever had in my life. It's only been six, five hours but it is definitely the longest, and I'm just glad it's done," said a relieved Kim, who also won a team silver for South Korea.
The South Korean men's soccer team can also skip military service thanks to Swansea City winger Eom, who swooped on a loose ball parried down by goalkeeper Rui Araki and smashed in a left-foot shot on the hour-mark.
Japan battled desperately to the finish, with midfielder Hisatsugu Ishii striking the post in the 79th minute, but the hosts were denied for a third successive final against the Koreans in the Under-23 tournament.
The Games has been blighted by organisational problems, drawing complaints from teams about accommodation and transport.
The women's hockey final was delayed by nearly an hour on Friday after the shuttle buses taking India and China's teams got stuck in traffic en route to the venue.
Japan's Olympic Committee president said Aichi-Nagoya organisers needed to reflect on their planning after they struggled to manage logistics for 17,000 athletes and officials.
The home athletes have done the country proud, though, and will finish with a national record haul of over 260 medals, smashing their previous best (218) at Hiroshima 1994.
China will top the table easily once again. They had 169 golds among 341 medals at the end of the day's competition, with second-placed Japan on 83 golds.
Coleman Wong won Hong Kong a first men's tennis singles gold and a provisional berth to the Los Angeles Olympics with victory over China's Wu Yibing.
