Kane, 18, who bagged an Easter double for the Lions over the weekend against Hull City and Portsmouth, has impressed during his three-month stint with the Championship side.
That takes his tally to seven goals from 20 starts since moving in January, five of which have come in the last 10 games.
His goals will be pivotal in Millwall's battle to avoid relegation, although Gallen is wary of complacency.
"I don't want to talk him up too much because he's decent," said Gallen.
"He's a very good finisher, very good all-round game and we're thankful to Tottenham for letting us have him.
"I think he's going to score lots of goals in his future. He loves football, he loves training – he gets annoyed when we pull him away from finishing training because we’re worried he’s going to get a thigh strain."
The striker had struggled for game time at Spurs, for whom he is yet to make a league appearance, and Gallen has hinted that, should Millwall avoid relegation, an approach would be made.
"He's got the lot," he said. "We've got to try and make sure we get some points and then we can concentrate on next season. It would be great if we could.
"We're very close to staying up, but not there yet."
By Andrew Gold
