The France prop, 23, joined from Catalans Dragons during the 2015 season, initially agreeing a two-and-a-half year contract.
However, having established himself as a fans favourite with his raw power and pace, the French Guiana-born forward has now extended his stay until the end of 2019.
Powell said: "Gadwin has worked exceptionally hard since he has been at the club to develop all aspects of his game.
“He is a big physical specimen whose ball carries are a real threat to opposition defence.
“I have no doubt he will continue to improve and develop into an all-round outstanding front rower of the future and I'm delighted that he has agreed to extend his contract at the club."
Springer, who has made 31 appearances, said: “I feel very happy to be staying at Castleford for two more years.
“Since I have come here I have improved a lot and I think of Cas as my second home now.
“It is great that the fans chant for me and I appreciate how they support the team. I just love playing at Jungle.
“We have a great side and I hope we can win something soon. I think that if we go through the season with no injuries we can achieve something great here at the Tigers.”