Skipper Paul McGinley was always expected to pick Poulter and Westwood, both of whom have outstanding Ryder Cup records and have shown sufficient form this summer to justify selection.
The remaining question mark was over the final spot, and McGinley went for Scotland's Gallacher - a former winner at the PGA Centenary course which will host the event, and a player who came agonisingly close to qualifying automatically, finishing third in the Italian Open at the weekend when a second-place finish would have clinched an automatic place.
Westwood, Poulter and Gallacher join automatic qualifiers Rory McIlroy, Henrik Stenson, Justin Rose, Sergio Garcia, Martin Kaymer, Graeme McDowell, Victor Dubuisson, Thomas Bjorn and Jamie Donaldson in McGinley's 12-strong team.
US captain Tom Watson will name his wildcard picks later on Tuesday.
McGinley said Gallacher, who would have displaced McDowell as the ninth automatic qualifier had he made it into the top two on Sunday, thoroughly deserved his selection.
"Stevie's first words to me when I told him he was in the team were, 'That's brilliant wee man'," McGinley said at Wentworth.
"Of course there's always a concern that he's a rookie but I'm not afraid to pick a rookie if he's good enough and there's no doubt Stevie earned his place in this team."
The Irishman said he was particularly impressed with Gallacher's display in Turin.
"His performances last week in particular had a big effect on me," explained McGinley.
"Stevie played relatively poorly for three weeks in a row in America but came back and had a good performance in the Czech Masters and then played well last week.
"For me Friday afternoon was when he showed he really wanted to be a Ryder Cup player. To come home in 30 shots and get within touching distance of the leaders showed just how strong he is," added McGinley.
Bookies William Hill have made the strong-looking European team 8-13 favourites to win, with the USA 13-8 and a draw priced at 10-1.
Gallacher's selection means that former world number one Luke Donald will miss the event for the first time since injury kept him out in 2008, while Francesco Molinari - one of the heroes of the 'Miracle of Medinah' two years ago - Miguel Angel Jimenez and Paul Casey also miss out.
The match takes place from 26-28 September on the PGA Centenary Course at Gleneagles.
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