Gartside is keen to reduce the club’s massive £93million debts after posting losses of £35.4million recently.
Cahill has been linked with a £17.5million move to Tottenham in recent weeks.
Elmander is out of contract at the end of the season and Bolton is keen to sell the striker for at least £2million.
Gartside said: “If a Champions League team knocks on our door in January, that’s the best time to sell an asset because Champions League teams will pay the money.”
“Johan’s out of contract at the end of this season and, again, if someone knocks on the door in January then that’s obviously a consideration we have to make.”