Sussex missed out on setting a new domestic winning record as they suffered their first Friends Provident t20 defeat when being stunned by eight runs by Gloucestershire at Hove.
Previously unbeaten South Group leaders Sussex suffered a shock defeat at home to cost them a new domestic record of 14 successive Twenty20 wins.
The Gladiators took advantage of the hosts being without England one-day stars Luke Wright and skipper Michael Yardy as they notched just a second win of the competition.
James Franklin inspired the visitors with his career-best innings of 90 in Gloucestershire's 178-4, and that proved just enough as Sussex could only muster 170-9 in response.
Having been inserted by Sharks' acting captain Murray Goodwin, Gloucestershire promoted pace bowler Jonathan Lewis to pinch hit and he responded with three boundaries in 10 balls before being yorked by Chad Keegan.
Kiwis Franklin and Hamish Marshall teamed up to post the Gladiators' 50 within seven overs as they went on to compile a second-wicket stand worth 91 in 63 balls.
Tall left-hander Franklin moved beyond the 1,000-run milestone in T20 matches as he cracked a 32-ball 50 with five fours and a brace of sixes.
Franklin hit 11 fours and three sixes in his 50-ball knock of 90 before Keegan hit his off stump in the penultimate over, while Marshall added a brisk 37 to the total.
Chasing down Gloucestershire's impressive 178-4 looked a tough task for Sussex with a run rate of nine an over, although they enjoyed two strokes of luck as both openers received a life from successive deliveries.
Matt Prior saw Marshall fail to hold on at deep-square and then, from the next ball from Steve Kirby, Brendon McCullum was downed by the same fielder at point.
Kirby responded and sent Prior packing for 17 with a yorker, but McCullum made slightly more of his let-off by cracking seven fours and a six in a 28-ball cameo for 46 .
Needing 93 from their last 10 overs, Sussex lost Murray Goodwin (31) to a sliced drive to cover and then crucially big-hitting Dwayne Smith, who lofted to deep square leg where Marshall made amends for his earlier blunders.
Chris Nash (one), in looking for a single, was run out by bowler Anthony Ireland when following through and, two balls later, Ireland had Andrew Hodd caught behind to finish with 3-35 as the Sharks lost their way in their frantic quest for late runs.