Needing 147 for victory, Delhi made heavy weather of their chase by failing to get partnerships going but finally got their target in 34.4 overs.
Juggling with their opening combination in the tournament so far, Gambhir (4), who struck a fluent 93 in the last match, came in with Unmukt Chand (2) but soon the score read 6 for 2.
Virender Sehwag, who also looked to be back in form after his smashing 80 in the previous game, threw away his wicket after a quick 20.
Rajat Bhatia top-scored with a patient 34 as wicketkeeper-batsman Puneet Bisht (15 not out) guided the team home alongwith Sumit Narwal (1 not out).
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Pace spearhead Suraj Yadav, new-ball partner Shadab Nazar and Nerender Singh picked up 2 wickets each.
Earlier electing to field, Delhi bowlers, led by the in-form Tokas, hurt Services by picking up regular wickets to bundle the Yashpal Singh-led side for a paltry 146 in 40.1 overs.
Pace spearhead Narwal started the rot by picking up Services opener Pratik Desai (19) and wicketkeeper-batsman Nakul Verma (0) on consecutive deliveries to be on a hat-trick, only to be foiled by Rajat Paliwal (12).
But Tokas it was who troubled the Services batsmen time and again to pick up his second consecutive four-wicket haul after he registered figures of 4-41 in Delhi's previous win against Haryana by the same margin.
Delhi with three wins from five matches have a strong chance of moving ahead from their group while Services have won two and lost three of their five games.
Brief Scores:
Services 146 all out in 40.1 overs (Devender Lochab 40, Ashutosh Aman 22; Vikas Tokas 4-43)