Sunderland claimed their first away win of the season in emphatic style, with Marcus Stewart's hat-trick doing the damage to Gillingham.
George McCartney and Stephen Elliott had both had chances blocked before Stewart netted the first, and Elliott's header doubled that early lead.
The third goal of a frantic start to the match came when Stewart followed up his own shot which had hit Elliott.
Stewart made it 4-0 when he intercepted a weak back pass to keeper Steve Banks.
Gillingham boss Andy Hessenthaler:
"They were better than us, it's as simple as that. It was schoolboy defending for their goals and that's not acceptable.
"The harder we tried the worse it got. But what's done is done, it's tough to lose in football but we're just going to have to take it on the chin.
"Normally we have one spanking a year and that one will be ours for this
Sunderland manager Mick McCarthy on hat-trick hero Marcus Stewart, who was nearly left out of the team:
"If my players have got a problem with a decision I make, I tell them to say it on the pitch. He 's done that.
"He was very good in training and I said to him to take that into the game and he was great.
"It was not just his goals but the manner in which he played. He ran their back four ragged."
Gillingham: Banks, Cox, Hope, Leon Johnson (Rose 81), Hills,
Hessenthaler (Agyemang 66), Smith, Spiller, Jarvis,
Roberts (Crofts 75), Byfield. Subs Not Used: Brown, Sidibe.
Booked: Leon Johnson, Spiller.
Sunderland: Poom, Wright, Breen, Stephen Caldwell, McCartney, Whitehead, Robinson (Oster 72), Whitley, Arca,
Elliott (Lynch 87), Stewart (Johnson 75). Subs Not Used: Collins, Myhre.
Goals: Stewart 4, Elliott 17, Stewart 20, 68.
Ref: A Hall (W Midlands).