Interestingly, last night I ran into a Kings fan from LA at a bar during the Canucks/Kings game, and he told me Gretzky's impact was immediate and explosive, and since then there are tons of hockey families in LA...to the point that California is now producing some great hockey players. Even his son plays hockey! He attributed it all to Aug 9th, 1988, the day of the "trade" (sale).
Popularize is the wrong word, he turned it from nothing into a niche phenomenon.
There was a lot of hype when he was traded, a lot of people took an interest but when he left or rather when the novelty had worn off people jumped off the bandwagon again as they tend to do in a place like SoCal. Some stayed on and those are the hockey fans one can thank Gretzky for, but it's obviously a very, very small % of the California population.
I grew up in California. The hockey boom was real and Gretzky was entirely responsible for that. He wasn't our guy in the Bay Area but the Sharks probably don't come to the Bay Area without him.
Obviously it faded - just as Gretzky did, and there was also the first lockout and the dead puck era - but it definitely left interest in California hockey from mites to the NHL higher than it had been when Gretzky arrived.
I grew up in LA and remember going to Kings games before Gretzky. The Forum would be regularly half empty, and games on tv were scarce. There was likely very little if any media coverage. Once Gretzky got there, Kings games were nightly featured on the news, attendance shot up to the point that the Forum was sold out almost nightly, and ticket prices tripled. What Gretzky's trade did on a national scale was even more dramatic. It brought hockey to (at least) the fringe of the mainstream. The Kings popularity in LA stalled after he left, partly caused by 17 years of mediocre (or worse) play by the club, but if Gretzky never came, LA's hockey following would be MUCH smaller than it is today.