TMZ Sports has learned ... the #99 jersey Gretzky rocked during the title-deciding game against the Boston Bruins is currently on the block at Grey Flannel Auctions ... and already has a bid over $400K!!

The craziest part ... there are still 18+ days left in the auction, and bidding doesn't typically heat up until the final day. In other words, this jersey is going to sell for an insane amount of money.

Wayne wore the iconic threads as he hoisted the best trophy in sports after leading his squad to a sweep over Boston in 5 games.

Yes, a sweep in 5 ... in a best of 7 series.

Gretzky and the Oilers had a chance to close out the Bruins in Game 4 ... that is, until the power went out with about 3 minutes remaining in the 2nd period of the potential series-clinching game.

NHL and team officials conferred, and ultimately decided to essentially throw out game 4 ... and just play game 5 two days later.

The Great One -- the NHL's all-time leading scorer -- lived up to his name during the 5th game ... finishing the game with 1 goal and 2 assists.

The jersey is a huge deal because it was the last time Gretzky ever donned an Oilers uniform ... as he was traded to the Los Angeles Kings a few months after winning the Cup -- in a shocking deal.

GFA tells us the jersey, a Nike size "XL," is covered in champagne stains from the boozy locker room celebration. The jersey also shows puck marks and loose threads.

The front features the Oilers team logo -- along with a "C" patch for captain -- located on the upper left chest area of the jersey.

The back of the uniform has Gretzky's name in blue tackle twill on a white nameplate, along with his iconic number 99 ... which has since been retired leaguewide.

GFA says the jersey was authenticated using photo-matching to ensure it's authentic.

Gretzky, who broke into the NHL in 1979, won the Stanley Cup 4 times and the Conn Smythe award twice, given to the most valuable player during the playoffs.