Simone Inzaghi's side are provisionally ahead of Napoli after netting all five of their goals in the opening 45 minutes at the Stadio Bentegodi through a Marcus Thuram brace and further strikes from Joaquin Correa, Stefan de Vrij and Yann Bisseck.
Napoli coach Antonio Conte criticised the Video Assistant Referee (VAR) for not intervening to reverse a penalty that Inter Milan's Hakan Calhanoglu eventually missed in their 1-1 draw on Sunday.
The Italian champions remain undefeated in the competition this season and sit fifth on 10 points at the midway point of the new league phase format.
Inter are now in second place in the table with 24 points, one behind leaders Napoli, while Venezia are 18th.
Manager Thiago Motta acknowledged that Juventus are not in the Serie A title race just yet and emphasised the importance of maintaining focus on their growth after they secured a dramatic 4-4 draw with champions Inter Milan thanks to two late goals on Sunday.
Argentina striker Martinez lashed home his fourth goal of the season in all competitions on the hour mark after a poor clearance from Zeki Celik to give Inter a hard-fought win at the Stadio Olimpico in Rome.
Defender Gabbia rose highest to thump home Tijjani Reijnders' free-kick in the 89th minute and give Milan a Serie A win which pulls them out of a miserable early season slump.
Henrikh Mkhitaryan blazed over the best chance of the match 15 minutes from time.
Inter and AC Milan on Friday rejected the project to modernise and restructure the iconic San Siro stadium which they share, city mayor Giuseppe Sala announced.
Inter Milan thrashed Atalanta 4-0 on Friday to move provisionally top of Serie A, while Torino snatched a late 1-0 win at Venezia to continue their positive start to the campaign.
"Winning is hard but staying up there is even harder. We had to bounce back after drawing at Genoa and we managed to do that," said Darmian to Sky Sport.
Inter Milan were denied a win in the opening match of their Serie A title defence on Saturday as late penalty drama left the champions with a 2-2 draw at Genoa.
Lautaro confirming his intention to stay at Inter is a huge boost for a club which headed into the close season in uncertain mood after a sudden change in ownership.
"Now, I feel like crying because we worked so hard, suffered so much, we deserved this joy," the 26-year-old told DAZN.
Inter Milan's Scudetto is easily Inzaghi's biggest honour since winning it as a player with Lazio 24 years ago, and comes after three years of off-field turbulence and serious financial problems which he has navigated with aplomb.
Goals from Francesco Acerbi and Marcus Thuram gave Inter a 17-point advantage over closest challengers Milan and secured their 20th league crown with five matches of the season remaining.
Noah Okafor salvaged a hard-fought point for Milan in the 84th minute on a hot spring day in Reggio Emilia, but second-placed Milan still risk conceding the Scudetto to Inter in a hotly-anticipated derby despite coming back from two goals down.
Italy midfielder Frattesi poked home the decisive goal in the fifth minute of stoppage time in Udine to maintain Inter's 14-point lead at the top of the table with seven matches left in the season.
Monday's comfortable victory left Inter needing just 11 more points from eight matches to beat second-placed AC Milan to a 20th Italian league crown.