Set just 126 to win, six-time champions Australia reached their target with a mammoth seven overs to spare to remain undefeated at this tournament heading into Sunday's final at Lord's.
Off-spinner Gardner struck twice in an over on her way to figures of 2-13 in the West Indies' meagre 125-7.
And Gardner, the player of the match, then made 35 not out in the chase, with opener Beth Mooney unbeaten on 61.
"A bit of momentum always helps," said Gardner at the presentation ceremony. "We knew it was going to be a nice wicket and really pleasing to get the win.
"I feel like I haven't had an impact with the ball in this tournament, so to save my best until now was really pleasing."
Tuesday's meagre semi-final target was unlikely to challenge an Australia side who had achieved a tournament-record chase of 171 while knocking out India in their final group game.
But the West Indies were left thinking of what might have been after experienced batter Deandra Dottin appeared to faint while the teams were lining up for the national anthems and had to be helped from the field.
Listed to bat at number five, she came in at eight with the West Indies deep in trouble at 83-6.
Dottin's 26 not out, off 16 balls including four fours - a mixture of elegant drives, scopps and ramps - at least got her side to a total beyond 100.
But by that stage the West Indies had already been derailed by a collapse that saw four wickets lost for 12 runs in 17 balls as they slumped to 59-4.
"She (Dottin) is a massive fighter and someone you never see go down," said West Indies captain Hayley Matthews.
"To see her almost passing out like that after the anthem was a bit scary. A bit of a medical emergency, but it shows how much of a fighter she is that she came back out and threw a few punches at the Aussies."
There was a worrying moment for Australia when Ellyse Perry retired hurt with a thigh problem at the end of the seventh over.
But Australia captain Sophie Molineux had no doubt that the veteran all-rounder would be fit for a final against either unbeaten hosts England or South Africa, who meet in Thursday's second semi-final at the Oval.
"Just some quad tightness," said Molineux of Perry. "It sounds like she'll be more than OK to go on Sunday.
Reflecting on her team's performance, Molineux added: "I thought we bowled really well.
"We held our nerve, and then we wanted to get it done as quickly as possible, and Moons and Ash were clinical."
Perry's exit made little difference to the outcome of the semi-final with Gardner sealing victory with an elegant flick off her legs for four.