This was pretty well known 30 years ago, but I don't know if people still know this.
When he was a kid, Gretzky often played on teams full of players who were a couple years older than he was. The jerseys were made for the older kids, so Gretzky's jerseys were often massively oversized on his slight frame. As a result, he got into the habit of tucking his jersey into his pants.
As he got older and turned pro, he continued to tuck in his jersey on the right side. This presented a problem for CCM and other jersey manufacturers because they placed their logo on the lower right part of the jersey. To overcome this problem, CCM would add a second logo to the lower left side of all of Gretzky's jerseys, just so people could see the logo while his jersey was tucked in.