I found the grumpy old man. Damn those zoomers for wanting gameplay that isn't chasing rabbits or getting stomped online, amirite?
GT is probably the biggest franchise if you count from 1997, simply by virtue of having an almost 25 year lifespan and a relatively large number of games released. There's a lot of history and simply no way for newer games to catch up with those sales figures when they're at an 70+ million unit deficit from the word go.
GT is probably not the biggest franchise if you look at sales per release in recent years. God of War is massive. Uncharted and Zero Dawn are both probably bigger per modern release.
GT Sport sold ~10 million units last I checked. God of War 2018 has sold more than 19 million on PS4 and is a top seller on Steam. Uncharted 4 sold 15+ million. Zero Dawn sold 10 million plus whatever it sold on Steam. Gran Turismo is big and makes plenty of money for Sony, but it's not Sony's biggest franchise in 2022.
At the moment it seems like Sony is choosing to put it's most popular franchises on Steam/PC as they hit end of life, presumably to entice PC users into the Playstation ecosystem for the newer releases. GoW and HZD are on PC already, Legacy of Thieves will be out later in the year. No talk of when or if Gran Turismo Sport will make it to PC.