Glamorgan, making six changes from the Championship, gave a debut to 18-year-old spinner Romano Franco, while experienced seamer James Harris played his first senior limited-overs game in three years.
Hampshire, with more extensive Hundred calls, had wicket-keeper Mayes, spinner Andrew Neal and seamer Lumsden making their first senior appearances.
Ali Orr and Gubbins gave Hampshire a useful start with 68 inside 14 overs despite a tight spell from Harris, who had Gubbins dropped at slip by Carlson early on.
Orr (45 off 44) hit Ned Leonard’s first ball for six but then drove him to Carlson at cover, and Indian international Tilak Varma lasted just three balls before edging Leonard to slip.
Gubbins held the innings together and found a willing fifth-wicket partner in England Under-19 star Mayes who played a wide range of shots from the off.
He cleared the ropes four times in a remarkably confident debut before holing out off Leonard, but Gubbins accelerated in the closing stages, also finishing with four sixes in his career-best score.
Felix Organ swung hard in a cameo of 32 off 14 balls as all Glamorgan’s bowlers suffered in the closing stages, Harris the pick with 1-51 while Kiran Carlson took 1-22 off six.
Mayes, who bowled seam for England Under-19s a month before, was quickly in action with the wicket-keeping gloves as Asa Tribe (14) edged Eddie Jack behind, while Will Smale lobbed Dom Kelly to cover in the next over.
Pacy Basingstoke-born Lumsden conceded 23 in an erratic first over as Carlson hit three sixes, but he had Eddie Byrom caught at square leg for 29 off the ninth and last ball, and then saw Billy Root chipping the first ball of his next over to mid-wicket.
Alex Horton hit 35 off 26 balls before falling to Varma, leaving everything dependent on Carlson.
He responded with some huge blows to all parts of the Gnoll, finishing with 10 sixes and nine fours, but 19-year-olds Kelly and Jack returned to take wickets and Carlson eventually fell to Lumsden to make the visitors’ victory secure.
