I know he was not draft by the Oilers but how did he enter to NHL? He was undrafted and he was signed by a NHL team like Martin St Louis?
He was signed as a free agent to the WHA Indianapolis Racers at 17 and then traded to the Oilers due to Racers finacial issues. The Oilers were then expanded to the NHL when the WHA was collapsing along with the Nords, the Jets and I think the Whalers.
Before Gretzky was eligible for the draft, he signed a contract with Indianapolis of the WHA at 17 (1978). He was not eligible for the NHL draft that season and the WHA was in the habit of signing underagers (Mark Howe was one of the first years before this).
Anyway, the NHL did make some noise about having him re-enter the draft when the merger came in 1979, but fact was that he was eligible in the one year that the merger happened (1979) and in all honesty there were questions about his durability and a bias toward the quality of the WHA.