The Frenchman ran comfortably in the front seven throughout the first half of the race, before the crucial shift to bald tires that the team needed to make when the track dried up.
But with the track not yet ready for dry tires, the team was called upon too early, which put Gasly well back in the standings. He eventually came home in 10th place.
After the race, Gasly said he didn't understand why AlphaTauri decided to gamble on the timing of the switch, especially because the team that stopped later gained more.
After the race, Guthrie said, "I don't think we did a good job. We were seventh, ahead of two Aston Martins, in the position on the track. We had everything at hand and we threw it away because we decided the box was too early.
"There was no communication, there was no dialogue, and I don't fully understand why. We took a gamble when we didn't need to. So we boxed, everybody stayed on track and we got passed four cars."
Guthrie said the end result, with late defenders such as Sebastian Vettel, Lance Stroll and McLaren's Daniel Ricciardo gaining a lot, was incredible because it opened the door for Aston Martin to overtake his team in the team standings.
"Obviously, I'm very disappointed for the whole team and all of us because we dropped to ninth in the team championship," he said. "Daniel was behind us, way behind, [but] not on track, as we should have been, and finished P5.
"We gave up nine points and an important nine points. So, yeah, I'm just sad. And not happy with the way we played."
Guthrie said that if teams had asked him about track conditions during stops, he would have advised them not to participate.
"It's too early," he said. "I don't understand why we did that. I haven't talked to them, and obviously it's something we'll review.
"They had a reason to do it, but at least we should have communicated, and that wasn't the case.
"In the position we are in, you are ahead of your two main competitors, you have track position in Singapore, the weather is dry and you know the warm-up is going to be tricky ...... I don't see the point of taking that kind of risk. We just gave up our position.
AlphaTauri's disappointment with the Singapore result was further compounded by Yuki Tsunoda's accident as he risked entering the track on slick tires and lost control on a wet section.
