Lopez won 12-11 on golden point in sudden death after the bout had finished level at 11-11 after the Scot had led 10-7 with 80 seconds left.
Sansum, a former kick-boxer, had mixed feelings after the contest.
“It’s my first medal at a Grand Prix,” the European silver medallist said, “While it’s not the colour I wanted, it’s good for the points and the road to Rio. I think I’m improving at a fast rate and still relatively new to the sport. I’m on track in where I want to be and comfortable in taekwondo now.
“The sudden death could have gone either way and I could have come home with gold but I’ll settle for bronze.”
Three convincing wins earlier in the day, including victory over world champion Tahir Guelec, pitted Sansum against Lopez in the semi-final.
The five-time world champion took a 4-3 lead into the second round after a quick start from both fighters, but was pegged back to 6-6 going into the final round. In a frantic finale, Sansum scored a headshot mid-way through the round to give himself the advantage but it was not long before Lopez responded with one of his own.
With the scores tied 11-11 and just seconds to go, a Kyong-go against Lopez put Sansum in the lead. But an appeal from coach Jean Lopez was successful and, instead, it was Sansum who was penalised and now trailing.
With just a second left, a body shot from Sansum scored, taking the bout into overtime amid huge controversy. Questions arose on whether time had expired when the kick had scored and a mistaken challenge on behalf of Sansum, who hadn’t realised he was now behind, and thought he had claimed victory. The bout went to golden point and after a tense round it was the American who prevailed.
“It’s the second time I’ve fought Lopez and it was a tight fight,” the 25 year-old Scot said. “The first round wasn’t the best but after that it was good. I caught him with a headshot in the clinch which is something I’ve been working on. I felt I could control the fight from then on, but fair play to him he went with the check and as I went to close he got me with a chop.
“In sudden death, I registered a couple of times, he registered a couple of times, but it was one of those that could have gone either way.”
Meanwhile, Olympic champion Jade Jones is edging closer to a first World Grand Prix title and hopes to end her quest in Manchester later this year.
The 21-year-old Welsh star won Great Britain’s third medal in Kazakhstan but was denied gold by Eva Calvo Gomez.
Spaniard Gomez had also got the better of London 2012 golden girl Jones in the first ever WGP at Manchester Central last December.
“I came here for the gold so obviously I am gutted,” said Jones after their latest -57kg show down. “But I feel like I have improved a lot, especially beating the world champion [So-hee Kim] by 12 points. Anything can happen with golden point. But I am chuffed with my improvements going towards Rio.
“Eva has become a great rival and this time she was the better fighter on the day. Hopefully, though, I can make it third time lucky in Manchester.
“I will go back to the gym now and work hard. There’s still two years to Rio and I know there is a lot more in the tank.”