After a battling first half of the race in which he kept Mercedes' Valtteri Bottas behind, Gasly looked to be in with a chance of sneaking into the points before rain started to fall.
As those around him pitted for intermediates, Gasly remained on slicks, a decision that left him frustrated with his second successive no-score.
"I think overall we did a very good first stint keeping Bottas behind for 30 laps," said Gasly.
"In the second stint, we were right behind Bottas before the rain came. They pitted and we decided to stay out, they finished P5 and we finished P13. We lost quite a lot there and we will revise what we could have done better.
"The pace was strong in Monza, it was again strong but we didn't capitalise on it.
"I don't think we deserved points because we made too many mistakes and it is such a difficult sport where you need to get everything to perfection and we didn't deserve it because we didn't do a good enough job. We will work and come back stronger."
To compound his misery, Gasly was hit by Lance Stroll in the dying stages of the race as the Aston Martin struggled for grip when rejoining from the barriers at turn seven.
"I think he went in the wall and came back and then in turn eight, clipped my rear wheel," explained Gasly.
"I haven't seen the footage but I turned, he touched me and basically I spun the car."
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