The late DC sports host Ken Beatrice used to say, “If Wayne Gretzky can be traded, anyone can be traded.”
The interesting thing is that Gretzky's trade was a business move, not a hockey move. A few yeas earlier, team owner Peter Pocklington was trying to sell off a portion of the team through an IPO, making it publicly traded. This is where his personal-services contract with Wayne Gretzky became an impediment -- because he couldn't list Gretzky as an asset of the Oilers unless he was under contract with the team instead of under contract with Pocklington. As a result of this change, Gretzky's new contract was more favorable to the player and gave him an opportunity to leave as a free agent much sooner than under the old one.
Pocklington was basically forced to trade him because he couldn't risk losing him in free agency and getting nothing in return. It was the same thing that drove the Oilers to trade all of their other big stars of that era -- including Mark Messier, Jari Kurri, Grant Fuhr, Glenn Anderson, and Kevin Lowe.