The Belgian striker quickly made an impact on his return from injury, guiding a deflected shot in from close range after 34 minutes.
Matteo Politano bundled in a second after the break to seal a much-needed win.
The three points will come as a relief to Gattuso, who has faced criticism following a run of three defeats in five Serie A games and a Europa League last-32 exit to Spanish side Granada.
The scoreline could have been more convincing, as Piotr Zielinski's first half strike was disallowed for offside and Kalidou Koulibaly could only head wide from six yards.
It was a night to forget for the Senegalese defender, who was sent off for a second yellow card on the 81st minute after a clumsy challenge.
Napoli move ahead of Lazio into sixth place, but both sides have 43 points, while Benevento are 16th with 25 points and without a league win in eight matches.