Both Gasly and team-mate Esteban Ocon were eliminated in Q1, qualifying 17th and 18th respectively for the Italian GP.
They didn't fare much better in the race, with Gasly just avoiding being lapped by winner Max Verstappen in 15th, while Ocon's car was retired as a pre-caution after he started encountering a steering lock issue.
While Gasly explained that Alpine had expected to struggle in Monza, that didn't make its weekend any less of a blow.
"It's been super painful," Gasly lamented. "I don't think we have ever had such a lack of competitiveness compared to our rivals.
"We knew straight away from the start of the season that it would be a painful one. But yeah, it was clearly very difficult out there."
Alpine's nadir came just a week on from Gasly scoring a podium in Zandvoort, where the Frenchman outpaced the Ferraris who now battled for the podium themselves.
It provided more evidence of Alpine's trend of struggling on lower downforce circuits, which Gasly hopes the team can understand so its weaknesses can be ironed out for 2024.
Reports of Alpine's power unit being 30bhp down on its rivals' engines is one possible explanation, but is unlikely to be the only factor, with the team also lacking some efficiency.
"The most important thing is really to understand and quantify where that drop of in performance is coming from and come back next year with a stronger package," added Gasly.
"Because I'm going past Carlos [Sainz] last week on track, same tyres, on pure pace, and this weekend he's standing on the podium, and I'm almost a lap down.
"We knew it's a very power sensitive track, but still, it seems like it's not only [that], it's the whole package, which has got to be better for this track."
"I think we will have another update coming quite soon. There is clear trend in terms of the type of tracks where we are more and less competitive.
"And I think that's something we need to work on for next year, to just have a package, more rounded package which works everywhere."
Ocon added the one horror show weekend in Monza doesn't take away from the progress he feels the in-flux Enstone team has been making behind the scenes and is no reason to allow heads to drop.
"I've had the worst ones in the past," Ocon said as Alpine remained a lonely sixth in the constructors' standings.
"It's not because we have a bad weekend, that we are going to give up. I think it's a good [weekend] in terms of learning and looking at the difficulties that we had.
"There's going to be plenty of analysis and we have a week when we're not racing, so we can focus exactly on what went wrong and come back from in Singapore."
Additional reporting by Adam Cooper
