Taking charge of his first game with the Rossoneri, Milan were denied all three points after Brignoli popped up with a late header to earn the home side their first points of the Serie A campaign.
Milan had led through Giacomo Bonaventura before George Puscas equalised, with Nikola Kalinic restoring the lead for Milan.
Gattuso has admitted the Brignoli goal was a blow but he would not slam his players for their efforts at the Stadio Ciro Vigorito.
"We have a long way to go and a lot to improve," said Gattuso. "We are licking our wounds. We will work to improve the fitness levels and mentality, but I cannot reproach my players for anything today."