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The 28-year-old survived a puncture with six kilometres to go, having broken away earlier on the final lap.
The way I felt those first couple of laps I just thought I'm going to DNF [Did Not Finish] let alone medal or win, so it's a dream come true
Thomas, who won Olympic gold on the track for Team GB in 2008 and 2012, said winning in a Wales jersey in Glasgow was special.
"It's up there alongside the Olympics, and everything to be honest," he said.
"I've only got to race for Wales maybe once before in the whole of my senior career, so it's a great feeling."
Earlier in the Games, Thomas won bronze in the time trial, just four days after completing this year's Tour de France for Team Sky.
Many of the 140 starters in the road race dropped out before the end of the 12 laps of the 14km circuit, in wet conditions, around the centre of Glasgow.
The Cardiff cyclist admitted he was nearly one of them.
"The way I felt those first couple of laps, I just thought I'm going to DNF [Did Not Finish] let alone medal or win, so it's a dream come true," he said
"It's massive. I think riding the Tour before, it just added to how good it feels because I really didn't expect it (to win).
"By the end I still didn't feel great but I just felt strong and like I could ride all day."
Thomas slipped down the field early in the race following problems with his bike's chain.
He fought back and then hit the front with England's Scott Thwaites (bronze) and New Zealand's Jack Bauer (silver).
The Welshman made his decisive break while going up the climb on Montrose Street.
"I knew I had to get a gap and time trial it and I thought I'd take them by surprise even though it was a long way out," said Thomas.
"I thought as long as I got a gap, they'd look to each other and just commit and fortunately I had the legs to go to the line."
On the final lap Thomas looked to be cruising to victory when he suffered a puncture and had to stop to have his front wheel changed.
"I couldn't believe it. I just felt it [the tyre] going down and I was like: 'oh no what's going on?'," said Thomas.
"I was definitely sweating. It just felt like it took an eternity to fix but it was probably relatively quick.
"Fortunately it didn't make a difference."
Cyclist Geraint Thomas said he was "surprised" after becoming the first Welshman to win Commonwealth gold in the men's road race.