Gloucestershire 254-8 (50 ov): Marshall 74; Roland-Jones 3-36, Fuller 2-45
Middlesex 211-3 (31.3 ov): Stirling 125*, Gubbins 37; Miles 1-26
Gloucestershire (0 pts) lost to Middlesex (2 pts) by seven wickets on D/L method
Paul Stirling hit a fluent century as Middlesex inflicted defeat on Royal London One-Day Cup holders Gloucestershire under the Duckworth-Lewis method at Bristol.
The home side posted 254-8, Hamish Marshall top-scoring with 74 and Gareth Roderick contributing 64.
Their stand was worth 114 in 127 balls but, once both had departed, Middlesex checked their progress with regular wickets.
Toby Roland-Jones (3-36) was the pick of the Middlesex attack, while James Fuller claimed 2-45 on returning to his former county.
Facing a revised target of 208 from 35 overs following a rain break, Middlesex skated to victory with 21 balls to spare.
Ireland batsman Stirling was dropped on one by Michael Klinger in the third over and that proved to be costly for Gloucestershire.
He reached his ton off 87 balls, with 11 fours and three sixes, before finishing unbeaten on 125.
Dawid Malan contributed 29 to an opening stand of 84 before being well caught at deep extra cover, while Nick Gubbins made 37.
Eoin Morgan perished at the death, but Stirling batted through and hit the winning boundary.