Sanchez is out of contract at Arsenal at the end of the season and is keen on leaving the Emirates.
Manchester City have had a £50million bid rejected for the 28-year-old but are set to test the Gunners’ resolve with a higher offer.
Arsenal attempted to get winger Raheem Sterling in a swap deal for Sanchez but City say Sterling will not be leaving the club.
Gale believes Arsenal should cash in on Sanchez instead of keeping him against his wishes and letting him go for free next summer.
The former West Ham striker says Arsenal should have a replacement lined up but doubts they do with all the current unrest at the club.
“It’s very strange with Arsenal because you would think they would have someone in mind,” Gale said on Sky Sports.
“You would think they would have a go-to player and had that player lined up as well.
“Have this deal lined up, £50million or whatever it is for Alexis Sanchez, and get one ready to come in.
“But I’m not sure with Arsenal at the moment, because all the unrest with Arsene Wenger in the season before, he didn’t know if he was signing his contract, Arsenal didn’t know whether they were going to keep him on.
“I think what that’s done has put all the recruitment in a position of ‘what do we need? When do we need them?’.
“I think Arsenal are very underprepared this season.”