Middlesex clinched their fifth win of the competition with a brilliantly measured chase to add insult to injury for Durham, who had England opener Gay retire hurt earlier in the day.
Durham posted 303 from their 50 overs and, although Gay left the field unable to continue with a shoulder complaint on nine, his opening partner Alex Lees added 103. Will Rhodes also contributed 65, while Middlesex bowler Naavya Sharma claimed figures of 4-64 from his 10 overs.
Chasing a daunting target, Middlesex put runs on the board early.
When Josh De Caires (34) got out to Kasey Aldridge, Middlesex captain Ben Geddes (77) and Joe Cracknell (84) immediately picked up the baton and put on 96 for the third wicket.
Neither were there to steer Middlesex over the line, but late hitting from tail-enders Toby Roland-Jones and Henry Brookes finished the job with just three balls left.
Yorkshire all-but sealed progress from Group B with a five-wicket win over Derbyshire at Scarborough, thanks to substantial contributions from batters James Wharton and Sam Whiteman.
Openers Harry Came (58) and Martin Andersson (67) shared a stand worth 140 for the Falcons before the hosts fought back.
Ben Cliff (3-57) led the way with the ball to ensure Derbyshire were made to settle on 278-8, and that made for an enticing out-ground chase.
Derbyshire appeared well placed to defend that figure when they had Yorkshire at 57-3. But century partnerships between Wharton (75) and Whiteman (94), and then Whiteman and George Hill (63 not out) carefully guided the hosts to victory in the final over of a gripping match.
Essex edged a thriller, winning by just three runs against hosts Glamorgan as more than 600 runs were scored and 17 wickets fell at Sophia Gardens.
Put in, Essex openers Benkenstein and Noah Thain (76) put on 166 for the first wicket, as the former smoked 140 from just 109 balls.
Having been 257-2, Essex collapsed and were bowled out for 312 with 17 deliveries left of their innings. However, they were well placed to defend that when they reduced Glamorgan to 78-5, with Shane Snater (3-49) making in-roads with the ball.
A terrific unbeaten 118 from Timm van der Gugten, well supported in the middle overs by Alex Horton (85), hauled Glamorgan towards the victory line.
Yet they fell short when needing six from the final over as Mackenzie Jones restricted them to just two.
Fynn Hudson-Prentice top scored with a blistering 68 from only 35 balls as he helped Sussex up to 333-9 – but Worcestershire Rapids won the day at Hove thanks to their skipper.
Hudson-Prentice, in his final season with Sussex before he joins Hampshire in 2027, was one of eight players in the batting order to pass 20 in a true team effort which failed to bear fruit.
Ben Gibbon (3-74) was rewarded for his efforts with the ball, but Jack Leaning also posted 50 as the hosts set an imposing total for their visitors to chase.
Isaac Mohammed (44) and Brett D’Oliveira (31) set the platform for the hunt and then Kashif Ali smashed 73 from 51 balls, including six fours and three sixes.
Libby played a captain’s knock to anchor the chase, finishing on 109 not out as the Rapids chased down their target with 10 balls remaining.