The Gloucestershire chased 171 runs target with six balls to spare after PS Handscomb completed his half century scoring 54 runs off 35 balls. Handscomb smashed three sixes and five fours.
The host also capitalized on Hammond's 38 runs from 28 balls and Brandon Gilmour's 22 off 12 balls.J Barrett (12) and Lezar (18) completed the run-chase remaining not out.
Nepali bowlers had a good start as GH Roderick was caught by wicket-keeper Subash Khakurel off Karan KC in the second over with just six runs on board.
Nepali bowlers also took wickets at regular intervals, but Handcomb and Hammond came to the rescue with the highest 64 runs partnership between them, which was enough to thwart the visitors' chances of a victory.
Karan bagged the highest three wickets for Nepal, while Sompal Kami, Basanta Regmi and Sagar Pun took one wicket each.
Earlier, being asked to bat first, Nepal put up 170/4 in allotted 20 overs as skipper Paras Khadka remained not out on 73 runs facing only 54 balls. He cracked two sixes and nine boundaries.
Although, Nepal started its inning in a shaky manner losing opener Subash (3 runs) in the first ball of the second over when there was only 12 runs on board, four other batsmen scored in double digits to anchor the innings.
Another opener Sagar, who scored 22 runs playing 32 balls, shared 24 runs stand for the second-wicket with Vice-captain Gyanendra Malla (14 off 13), who was caught by Robbie Montgomery off Miles Hammond in the sixth over.
Sagar, who hit a six and two fours, was also involved in the second highest partnership of 49-runs with skipper Paras for the third wicket.
Paras anchored Nepal's inning sharing highest 65 runs stand with Sharad Vesawkar (12 off 14) for the fourth wicket.
Pradeep Airee remained not out on 16 runs facing only six balls that included a six and a four.
Hammond, Montgomery and Dan Lezar claimed one wicket each for Gloucestershire.
Earlier during the tour, Nepal had faced a 14-run defeat against Middlesex CCC 2nd XI before beating Free Forester by 25 runs.
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