Ollie Robinson took two wickets in three balls as Sussex put themselves in a strong position to claim their first Vitality County Championship win of the season.
Having bowled 27 wicketless overs in the match, Robinson was finally rewarded when he trapped Cameron Bancroft and James Bracey leg before as Gloucestershire slumped to 27 for 4 in their second innings. They reached stumps on 81 for 6, leading by just 19 at Hove.
Ben Charlesworth pulled the third ball of the innings over mid-wicket for a six, but Robinson quickly settled into an impressive rhythm at a good pace down the slope and was rewarded in his sixth over. Bancroft played a cut-back that would have hit leg stump and Bracey, showing minimal footwork, was plumb in front of a straight one.
Robinson's workload is managed by the ECB. His 9-3-21-2 was his longest - and best - spell of the season so far. He bowled four more Test overs and had 2 for 32 from 13 overs at stumps - with power to add tomorrow.
Jayden Seales had taken two wickets in his first two overs after Gloucestershire resumed on 62 in their first innings, having bowled out Sussex for 479.
Charlesworth drove a loose drive to backward point in the West Indies' first over and Ollie Price made a fatal miscalculation in his next when he left a length ball that uprooted his off stump.
Skipper Graeme van Burren and Miles Hammond made a brief recovery before off-spinner Jack Carson struck with his fifth ball, van Burren missing an attempted pull at one which was kept low.
James Coles then gave Sussex a sixth wicket in the penultimate over when Tom Price was leg before.
Earlier, Cheteshwar Pujara, John Simpson and Danny Lamb had all passed 50 before Sussex were bowled out for 479 and it was surprising that none of them made the first hundred of the match.
Pujara moved effortlessly from his overnight 75 to 86 and looked on course for a hundred before giving away his wicket. He drove the ball to deep extra cover and hesitated to go for a third run. Sub-fielder Jack Taylor's throw was accurate enough to allow Bracey to run him out.
When Fynn Hudson-Prentice was stumped in flight by Zafar Gohar, Sussex still trailed by 103 runs, but Simpson and Lamb tipped the balance with a measured stand of 143 in 34 overs, breaking the previous county record of 122 for the eighth wicket against Gloucestershire set by George Brann and Walter Andrews in 1892.
Lamb was the more aggressive, but Simpson, who made his maiden double hundred against Leicestershire last week, reached his fifty by lofting Gohar over long on and added eight fours before misjudging a pull and Hammond running in to take a fine catch at midwicket.
Lamb, who scored a century at Leicester, continued his fine form with a 116-ball 83 that included nine fours and a six. Trying to force the pace, he was bowled by Don Goodman trying to drive him over the wicketkeeper.