James Milner scored the winner from the spot in the 79th minute at Priestfield on Sunday after Ashley Young had gone down in the area.

Simeon Jackson had earlier turned Zat Knight and smashed home a 57th-minute equaliser to cancel out Milner's first-half opener.

Villa boss O'Neill told ITV1: "It was really tough. I came here last week and knew it would be tough. If the players didn't know it would be tough they did after 10 minutes.

"They (Gillingham) are very strong. They played very well and good luck to them this season."

O'Neill praised his match-winner Milner, particularly for his well-taken opening goal, which he coolly slotted into the corner after a neat one-two with Stiliyan Petrov.

He added: "It was a terrific goal by James, very well worked, a nice goal indeed."

O'Neill and Gillingham boss Mark Stimson exchanged words on the touchline in the second half, but O'Neill played down the spat.

"We shook hands in the end," he said.

Gills frontman Jackson felt the League Two side had matched their top-flight opponents for long periods.

He said: "As a team we did very well. We showed we can compete. Unfortunately we didn't get a result."

Describing his goal, Jackson was modesty personified, adding: "It was ok. I tried to connect well with the ball and it went in."

Villa will be away to either Cheltenham or Doncaster in the fourth round.