The French international told the News of the World: "I have six months left to make my decision but I am 28 and maybe it is time to look elsewhere.
"Maybe I need a new challenge. I have been here since 2001 and of course I have enjoyed it.
"But I have got five or six more years before the end of my career and the choice I have to make is a huge one."
Gallas, who cost the Blues £6.2m when he arrived from Marseille, is under contract until 2007 but is already considering his next move.
He added: "There are some massive clubs out there who might prove tempting to me.
"I remember when a France team-mate told me once: 'When you wear the AC Milan shirt, it shows you have succeeded'. That makes you think."
After playing most of last season at left-back, Gallas began this campaign in his preferred position in the centre of defence.
However, he then played at right-back in Chelsea's win over Liverpool last weekend.
He is frustrated at being asked to provide cover across the Blues defence and is also unhappy at earning less than some of his team-mates.
"I know my strengths, my qualities, and I know what I am worth," he added Gallas.
"We have had a lot of success and I know what I can bring to this Chelsea team. Manager Jose Mourinho knows he can count on everyone in the side - and that includes me.
"But I have to decide about my future.
"We are looking to win the Premiership once again and maybe even the Champions League. It is a vital time for everyone concerned."