Nowacki, who has beaten some of Ramsay-Peaty's British junior records, finished ahead of the 2016 and 2020 Olympic champion in the 100m breaststroke final.
The Jerseyman also pushed Ramsay-Peaty all the way in the 50m final, finishing fifth behind the Englishman who won bronze.
Nowacki led from start to finish as he opened up a 0.72 second lead after 150m before coming home in a time of 2:07.87, beating Canada's Oliver Dawson into second place with Australia's Zac Stubblety-Cook winning bronze.
Nowacki's success has secured Jersey's first gold since Colin Mallett topped the podium in fullbore shooting in 1990.
But in a winning a gold and silver he has surpassed Mallett's feat in Auckland when he also won fullbore pairs bronze with his father Cliff.
"It's absolutely unreal," Nowacki told BBC Jersey after the victory.
"People don't understand that you just enter such a heavy flow state during the race, and all of a sudden you finish and you pop back into reality and the crowd comes in."
He added: "It's very special and I owe it to two groups of people.
"My family being my main support and my girlfriend, and also I think the big thing in English swimming is coaches often don't get along between boarding school and club coaches, so the second group of people I put that down to is my coaches."
Nowacki has just left Millfield School and will move to Loughborough to continue his training as he aims towards the 2028 Olympic Games.
He won two world junior titles last year, including the 200m breaststroke, and says he is happy with how he has adapted to the senior environment.
"That jump between junior and senior is huge and it's often overlooked.
"So I'm very pleased to have got my hand on the wall first.
"You don't often see swimmers make that jump straight away, so I'm very fortunate and grateful to have made that jump."
'A true ambassador for Jersey'
Nowacki is just the second person from Jersey to win a Commonwealth Games gold medal
The impact of Nowacki's achievement on his home island of Jersey should not be underestimated.
Competitors representing the Channel Island with a population of about 100,000 had won just four medals before these games.
Nowacki has increased that tally by 50% and become the first-ever swimmer from the island to win a medal.
"There'll be children watching tonight that will want to be like Filip, so it means everything to all of the team here and all of those people watching back home," Jersey's chef de mission Jean Cross told BBC Jersey.
"I think we'll see Filip in LA [for the Olympics], there's so many people talking about Filip here and it's just lovely to be associated with a swimmer of his talent
"He's a true ambassador for Jersey, he's done so well."
Nowacki becomes one of Jersey's greats
Image source, PA MediaImage caption,
Nowacki won Jersey's first medal since 1990 on Saturday
Nowacki's victories propel him to one of Jersey's greatest ever sports stars.
The Commonwealth Games is the highest level at which athletes from the island, with a population of about 100,000, can compete under their own flag.
Previous to Colin and Cliff Mallett, Jersey had won just two medals in their own right.
In 1986, bantamweight boxer John Sillitoe won bronze in Edinburgh, while Jersey's first-ever medal was also in the ring when Albert Turmel won welterweight bronze in Perth in 1962.
Islanders have won medals for England - diver Dot Macready won 10m gold in 1934, while more recently Serena Guthrie was part of the England team that won netball gold in 2018.
In 2006, Rebecca Herbert was part of the England hockey team that won bronze, while there were two bronze medals in 2002 for islanders representing England - Elizabeth Cann in badminton and swimmer Simon Militis in the 200m backstroke.
But with Nowacki aged just 18 and already a British senior and world junior champion few would bet against the swimmer going on to become the most decorated Commonwealth Games athlete in Jersey's history.
He is also part of the Team GB set up with an aim firmly on the 2028 Olympic Games in Los Angeles.
Jersey has not had an Olympic competitor since Militis went to the 2000 Olympics in Sydney and all island eyes will be on their young hero bringing them more glory.
