Gallen will turn 37 next month, and should he play on, would reach the 350-game milestone in a record 19th campaign next season.
It is believed that Gallen has changed his mind on several occasions in regards to his playing future, and at one stage had decided he was going to hang up the boots.
He currently holds the record alongside teammate Luke Lewis and fellow Sharks great Andrew Ettingshausen for the most seasons in the league.
The Cronulla coach believes Gallen will make a call when the time is right.
“He’s probably leaning on the side of playing again but he won’t make a decision until the end of the season,” Flanagan told reporters.
“If we’re lucky enough and good enough to play in a grand final I’d say he’d pull up stumps then.
“He won’t make a decision either way, he’ll just let it hang and make a decision at the end of the season.”
Flanagan added that Gallen’s current output, numbers and health still hold up with the best of them.
“If he’s playing like he is at the moment, he can play on,” Flanagan said.
“But he needs to want to train and do the stuff during the week.The game part of it he’s doing fine, his numbers are great.
“But it is an ask to do another pre-season, four months of slog.”
Flanagan also said his skipper would ultimately consider doing what is best for the Sharks, wanting to give back to the club that has nurtured him.
“The love of the game and the love of the club – Gal’s sort of said to me that a lot of his career has been personal achievements and winning the grand final was a massive tick off that bucketlist,” Flanagan said.
“But now he just wants to give back and bring the next generation of players on. “He said that if he did play, if there’s someone better than him, he’d be happy to stand down and move on.”