The Gunners' title challenge has again fallen away this season.
And Gallas told talkSPORT: "I'm a little bit disappointed to be honest about what Arsenal is doing this season, because for me, I think they didn't improve from last season.
"And this season, I think it was a great opportunity to win the Premier League.
"Mikel Arteta, I think, made some mistakes. Not only him, I think players make a lot of mistakes when they draw some games in December.
"And for me, I don't think only for me, but I think as well for the fans, it was very difficult to accept some games they drew.
"Especially against, for example, Everton at home, they lost at West Ham, they drew at Fulham and if you want to be champion, you can't make that mistake."
Gallas, on if Arteta is the right man for the job, continued: "What he did with Arsenal is great because Arsenal, maybe five years ago, they were in a difficult position and he built something with that squad and he did very well.
"Now the question is, can he win the Premier League? And at the moment, my answer is no. Because now he's almost, yeah, he's three times, he's going to finish second.
"This season, Manchester City was hard for some reason. I think they were tired, mentally it was very difficult for the players.
"And this season it was the best opportunity to win the league. And when you see Liverpool were in front, OK, they did well.
"But they had the new manager and how, as we know, sometimes it's very difficult for the new manager to arrive in the new club and to try to do his best for his players to be on the top.
"Sometimes you need some time. But with that manager straight away they got a good result. But Arsenal was not there."