Ishan Kishan was dismissed by Bjorn Fortuin, the left-arm orthodox spinner finding a faint edge that was taken by De Kock with India two wickets down for 58 runs.
Gill struck eight boundaries on his way to 49, but he was trapped LBW by Lungi Ngidi, having shared 39 runs with Shreyas Iyer for the third wicket.
Iyer (28) smashed a six off the first ball of the 20th over by Marco Jansen as he and Sanju Samson (2) ended not out to take their side home.
Earlier for South Africa, De Kock (6) fell early, caught by Avesh Khan off Washington Sundar, with just seven runs on the board before Janneman Malan (15) and Reeza Hendricks (3) fell within two Siraj overs as South Africa stumbled to 26 for 3 in 10 overs.
Heinrich Klaasen and Aiden Markram (9) added 17 before Markram was caught behind by Samson as Shahbaz Ahmed struck.
Captain David Miller (7) became Sundar’s second victim when he was bowled, having shared 23 with Klaasen and the visitors found themselves in all sorts of trouble on 67-5 at the end of the 19th over.
They lost their next five wickets for just 32 runs in 43 balls.
All-rounder Andile Phehlukwayo added just five before being bowled by Kuldeep Yadav, before Klaasen, who hit four boundaries on his way to 34, had his stumps disturbed as Shahbaz claimed a second.
Yadav trapped Bjorn Fortuin leg before wicket for one before bowling Anrich Nortjé for a duck one ball later to leave South Africa reeling on 94-9 in the 26th over.
South Africa offered little resistance for the last wicket as Marco Jansen, who hit a four and a six in his 14, was caught on the boundary by Avesh Khan to close out the Proteas innings.
Spinners rout Proteas as India wrap up series in New Delhi
Image: Pankaj Nangia/Gallo Images
India's spin bowlers took eight wickets between them to send South Africa spiralling to an embarrassing seven-wicket defeat in the third and decisive ODI in New Delhi on Tuesday.
After the Proteas were shot out for a dismal 99, their lowest ODI total against India, the home team romped to victory in 27.1 overs, losing only three wickets along the way.
Kuldeep Yadav was the pick of the Indian spinners, taking four wickets to clean up the visitors’ tail. Shahbaz Ahmed (2-32) and Washington Sundar (2-15) worked their way through the top and middle order along with seamer Mohammed Siraj, who claimed two of his own.
Left-arm wrist spinner Yadav ended with figures of 4-18 to consign South Africa to their fourth-lowest total since 1993 and wrap up the series 2-1.
In pursuit of the modest target, Shubman Gill was the aggressor as India raced to 42 inside seven overs before captain Shikhar Dhawan (8) was run out after some swift work in the field by Marco Jansen returning the ball to Quinton de Kock.
Ishan Kishan was dismissed by Bjorn Fortuin, the left-arm orthodox spinner finding a faint edge that was taken by De Kock with India two wickets down for 58 runs.
Gill struck eight boundaries on his way to 49, but he was trapped LBW by Lungi Ngidi, having shared 39 runs with Shreyas Iyer for the third wicket.
Iyer (28) smashed a six off the first ball of the 20th over by Marco Jansen as he and Sanju Samson (2) ended not out to take their side home.
Earlier for South Africa, De Kock (6) fell early, caught by Avesh Khan off Washington Sundar, with just seven runs on the board before Janneman Malan (15) and Reeza Hendricks (3) fell within two Siraj overs as South Africa stumbled to 26 for 3 in 10 overs.
Heinrich Klaasen and Aiden Markram (9) added 17 before Markram was caught behind by Samson as Shahbaz Ahmed struck.
Captain David Miller (7) became Sundar’s second victim when he was bowled, having shared 23 with Klaasen and the visitors found themselves in all sorts of trouble on 67-5 at the end of the 19th over.
They lost their next five wickets for just 32 runs in 43 balls.
All-rounder Andile Phehlukwayo added just five before being bowled by Kuldeep Yadav, before Klaasen, who hit four boundaries on his way to 34, had his stumps disturbed as Shahbaz claimed a second.
Yadav trapped Bjorn Fortuin leg before wicket for one before bowling Anrich Nortjé for a duck one ball later to leave South Africa reeling on 94-9 in the 26th over.
South Africa offered little resistance for the last wicket as Marco Jansen, who hit a four and a six in his 14, was caught on the boundary by Avesh Khan to close out the Proteas innings.
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