The 28-year-old won the race in comfortable fashion as he finished well ahead of the chasing pack in a time of 4:13:05 to secure his second medal of the Games.
Thomas told BBC Sport: "Winning is unbelievable. I never expected it. Coming off the tour I was tired at end of that. I thought it would be a massive challenge to get a result here but the motivation was there.
"I was buzzing to compete for Team Wales. I just got stuck in and it couldn't have got any better."