Did Wayne Gretzky ever wear a number other than 99 during his NHL career?

it does not appear so, 99 seems to be it.

https://www.nhl.com/player/wayne-gretzky-8447400

As an Edmontonian who watched him from the time he first joined the Edmonton Oilers while they were still in the World Hockey Association, a year before they were merged into the NHL, I can vouch for him wearing only number 99 for his entire NHL career for the Oilers, the LA Kings, the St. Louis Blues, and the New York Rangers. The entire league has retired number 99 in his honour, so no-one ever will ever wear it again. He wore 99 for the WHA Oilers, too- he began wearing 99 in Major Junior with the Sault Ste. Marie Greyhounds, and continued after that. As the story goes, he wanted number 9 with the Greyhounds, in honour of his hero, Gordie Howe, but that number was taken. He tried 14 and19 for a while, until his coach suggested doubling up on the 9s, and the rest is history. -see http://hockeygods.com/images/9244-Wayne_Gretzky___16_Years_Old___Soo_Greyhounds___1977 and https://en.wikipedia.org/wiki/Sault_Ste._Marie_Greyhounds

He was photographed wearing some other numbers for the Indianapolis Racers, where he played his first 8 WHA games before his sale to the Oilers, but he was wearing 99 for the Racers when he scored his first WHA goal- against the Edmonton Oilers!