Daryl Powell's side were universally acknowledged as by far and away the best team in the league in 2017, topping the table by 10 points from second-placed Leeds and finishing with a staggering points difference of +429, but St Helens deserve enormous credit for their part in a remarkable game in which they outscored their hosts by five tries to three.
Zak Hardaker crossed for the hosts in the first minute after a Zeb Taia fumble but tries from Regan Grace and Matty Knowles gave St Helens a 12-10 half-time lead.
Gale's try was followed by Adam Milner's in the 64th minute but it was the crazy finale to this semi-final which will live long in the memory.
Luke Gale: "Wow. It's hard to sum up, I'm emotional at the minute. It was an awesome game, I thought we'd blown it. It came down to the last second and that's my job to get those drop goals.
"It's pretty special being in a hospital bed two weeks ago thinking my season was over to this, but things happen for a reason."
Castleford coach Daryl Powell on Gale and his fitness concerns prior to the match: "On Monday he was pretty sure, I just wanted to give him until Wednesday. He handled it pretty well.
"It was a phenomenal effort to get himself on the field in such a short space of time, although he might have hurt himself being dived on at the end.
"He held his nerve with the penalty and that drop goal was world class. To get it over over the top of onrushing defenders was pretty special."
Powell on the game: "It was obviously really tight all the way through but we got a 10-point cushion and that should have been enough.
"But we were a little bit soft on our tryline. There were some disappointing things in there but it was unbelievable at the end and to come out on top in extra time is massive for us.
"We've done a Saints on St Helens. They'll be disappointed because they played well tonight. Goalkicking let them down but Percival has been an awesome player for them so I don't think you can point the finger too much at him."
St Helens coach Justin Holbrook: "It's really hard to take. It can be a cruel game at times - and it cost us.
"It will take some getting over. I'm obviously really proud of our boys, how we kept fighting, especially to come back in the second half."