England openers Charlotte Edwards and Arran Brindle put on 112 to help the hosts to 227 for seven, Sadia Yousuf taking three for 33.
Pakistan's top order offered resistance, but once Gunn took the third and fourth wickets in three balls, the remainder of the line-up folded and, in all, eight wickets fell in 10 overs for 16 runs.
After Edwards won the toss, she and Brindle both hit early boundaries but largely had to settle for steady accumulation. Their half-century stand came up in the 18th over, immediately followed by Edwards' third four.
Four singles in the 28th over took England to three figures but Brindle soon departed for 55 from 102 balls, stumped by Batool Fatima off Yousuf the ball after Edwards had followed her to a half-century.
Sarah Taylor fell to Yousuf for 15 but Edwards struck back-to-back fours off Nida Dar to bring up the 150 with 10 overs left.
She was finally bowled by Sumaiya Siddiqi for an 83 featuring seven boundaries, but Lydia Greenway made 38 from 29 balls to lift England's total.
Pakistan started steadily, if slowly, in response as Javeria Khan (39) put on 58 with opening partner Nahida Khan (23) and 35 with Bismah Maroof (20).
That took them to 101 for two but Gunn ripped out Javeria and Maroof with no addition to the score to spark the spectacular collapse.
- Sports & Recreation
- Charlotte Edwards
- Arran Brindle