Dettori was able to dictate the pace virtually from the outset on the Ribblesdale Stakes winner, from Peach Tree and Manuela De Vega.
Quickening three furlongs out, Star Catcher held a healthy advantage until the Aidan O'Brien-trained Fleeting put in a strong late effort.
However, Star Catcher pulled out more to win by a half a length. Pink Dogwood, also from the O'Brien stable, was four and a half lengths back in third.
Gosden, who won the Investec Oaks at Epsom with Anapurna, said: "When I suggested supplementing her to Mr Oppenheimer he was very brave and said, 'go for it if you think it's right', so he gave me full support.
Full Finishing list 2019
1st Star Catcher (J. Gosden,F. Dettori), 2nd Fleeting (A. O'Brien,D. O'Brien), 3rd Pink Dogwood (A. O'Brien,R. Moore), 4th Search For A Song (D. Weld,C. Hayes ), 5th Manuela De Vega (R. Beckett,H. Bentley), 6th Peach Tree (A. O'Brien,S. Heffernan ) , 7th Iridessa (J. O'Brien,W. Lordan), 8th Trethias (M. Harrington,S. Foley)
Distance: 1m 4f (2,414 metres), Surface: Turf, Track: Right-handed,Qualification: Three-year-old fillies, Weight: 9 st 0 lb,Purse: €400,000 (2019),1st: €228,000