Dennis had earlier won the gold medal in the men's individual time trial, but woke up with discomfort on Saturday morning.
AusCycling and the Australian Commonwealth Games team later announced the 32-year-old's withdrawal, leaving Luke Durbridge, Luke Plapp, Miles Scotson and Sam Fox to contest a road race ultimately won in a sprint finish.
Geraint Thomas looked to have done enough inside the final 1.8 kilometres but his late attack was nullified by Plapp, before Gate powered past his WorldTour rivals for his fourth gold medal at the Games.
Daryl Impey (South Africa) and Finn Crockett (Scotland) rounded out the podium, having witnessed their Kiwi rival overcome both cramp and nerves on the 160-kilometre course.
"I was definitely cramping up multiple times in those last two laps following all those moves," Gate said after the race.
"I was like ‘ooh, this one could just push me over the edge’, and it was too late to try and play catch-up with hydration at that point.
"I was a little bit nervous that when I started to open my sprint my legs were just going to lock up – but luckily I was able to drive the bike to the finish line.
"I was literally on the razor’s edge of getting to the line and I just couldn't believe it."
The Bolton Equities Black Spoke Pro Cycling rider was the only New Zealander in a group of 15 who escaped with 100 kilometres to go.
English trio Ben Turner, Sam Watson and Fred Wright did their best to break clear of the bunch, but it was Gate who continued to close each attack down.
Plapp was the next to play spoiler as the race approached its final 10 kilometres, but not before Watson and Sam Culverwell (Northern Ireland) went clear, almost undetected.
Thankfully for Gate, Impey and compatriot Morne van Niekerk were on hand to help drag the duo back, with Thomas the last remaining hurdle before the final sprint to the line.
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