Ryan Gosling played all the piano parts?

He's playing, but you're not actually hearing him. He learned to play those pieces over a 3 month period so that he could convincingly mimic it so as to avoid the use of a hand double, but the music was pre-recorded before anything was shot.

So you're seeing him play, but hearing someone else.

We weren't actually hearing him though, the recordings were Randy Kerber.

“Sure enough, in the last few days he switched gears and did a ton of work on the physicality and attitude of his performances. One of the first scenes in the shooting schedule was his first big piano solo in the club, and I’ll never forget the astonished silence which settled on the room at the end of his first take. It was kind of miraculous. What he played was a very musically intelligent approximation of the difficult sections, with perhaps a few wrong notes here and there, but the basic melodies he had totally down. So even though the piano parts you hear in the film are played by Randy, the visual representation, every single pianistic hand gesture, was Ryan’s. It’s an astonishing achievement that when you watch the movie, you don’t for a second question that Ryan’s character plays the parts.”

https://www.soundonsound.com/techniques/inside-track-la-la-land

They always embellish these things.

But.... ya better believe it in this case! :} Dude showed some skill.