Downs opened the scoring in the 25th minute through a header by Alje Schut after connecting with a cross from Teko Modise.
The Tshwane giants dominated most proceedings before the two teams headed for the changerooms.
Modise then missed a glorious chance to bury the Dube Birds in the 42nd minute when his short at goal was kept at bay by Swallows keeper Obren Cuckovic.
Downs coach Pitso Mosimane made a triple substitution replacing Alje Schut by Mario Booysens, Katlego Mashego for Rodney Ramagalela and Modise for Asavela Mbekile.
Swallows kept probing but to no avail.
The Dube Birds came close in the later stage of the match when Lerato Chabangu’s strike went wayward.
Ramagalela came one-on-one with defender Boris Savic and took no chances to miss the mark and score Sundowns’ third.
Themba Zwane his brace the first on from the penalty spot in the 73rd and the second in the 90th to book a place in the final.
The Gauteng Cup consists of two semifinals between Downs and Swallows, and the University of Pretoria and SuperSport United.
Downs will play the winner in the second match at 4.30pm.
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