Game & Watch was first, still Gunpei Yokoi, but yeah I’d consider that the start of mobile gaming - at least that’s my memory!
It was the first really successful portable system with interchangeable games and a dot matrix screen as opposed to custom LCDs per game like G&W.
The Milton Bradley MicroVision came first around 1980. Nintendo referenced it as a precursor. But the screen was 16x16 pixels so ‘graphics’ is kind of misnomer.
I think I remember seeing something once where the screen was included with the game, because it was a static LCD sort of like a Game and Watch game.
The GB was in the right place, at the right time, from the right company, with the right hardware and software, at the right price point. Instant success.
I think the Game & Watch gameplay feels a lot closer to an electronic toy from the early 80s than what people would consider console gaming.
Not to detract from its merit. It's just that the Game Boy wasich closer to a NES-like experience.
Just like a simple digital watch, everything that could be on the screen was designed upfront and then the individual bits could just be shown or hidden.
Nintendo eventually made three collections of the Game and Watch games for the GB. Each collection was a ton of fun with each game available in classic (faithful) or updated (graphics replaced with sprites, same gameplay) modes.