Gibson tore the ACL in her left knee shortly after kicking the opening goal of West Coast's upset 12-point win over Port Adelaide last Saturday.
The 26-year-old, who won a flag with Adelaide in 2017, was the first of two players to suffer an ACL tear in the opening round.
Brisbane defender Lily Postlethwaite ruptured her right ACL in innocuous fashion during the Lions' 49-point win over Fremantle on Sunday.
It was Postlethwaite's first AFLW match in 553 days following an ACL reconstruction to her left knee.
But there was better news for Sydney's No.1 draft pick Montana Ham, who is expected to return in four weeks after being cleared of an ACL tear.
The Swans feared the worst after Ham's knee twisted awkwardly, but scans have since shown she suffered a ligament strain rather than a tear.
Gibson was set to play a key role at West Coast this season, and her absence will be keenly felt by last year's wooden spooners.
Eagles forward Shanae Davison suffered an ACL tear during the pre-season, while key defender Evie Gooch was ruled out for the season with a wrist injury.
West Coast will be without last season's leading goalkicker Aimee Schmidt for Sunday's clash with Gold Coast at Metricon Stadium due to concussion.
"Unfortunately, scans have confirmed the worst and It's disappointing news for Kellie and the entire squad," West Coast's head of women's football Michelle Cowan said of GIbson's injury.
"We will wrap our arms around Kellie and support her through her rehabilitation.
"She is a proud and passionate Eagle, and we know that she will be doing everything she can to support the team for the rest of the season.
"While Kellie will not be playing this season, she will be still around the playing group and the club working alongside rehab physiotherapist Alex Chin.
"Kellie will also play a key part with the playing group and continue to evolve her leadership skills off the field."
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