While I am willing to accept the idea that gamebooks can be considered RPGs, although they occupy the design space that interests me the least in RPGs since there's no collaborative story telling.
And some gamebooks do have built in mechanics, for combat and such, which definitely gives them an RPG feel. In fact, Tunnel and Trolls has a long history of solitaire gamebooks, for lonely gamers.
Seriously, back in the days before computer RPGs (or, more importantly, portable computers like, say, smart phones), gamebooks were the best option if you couldn't find anyone to play with. Although I remember a friend recommending Role Master for gamers without friends so you could spend hours making a character without the heartbreak of of the critical hit table killing them in five minutes.
I do have some found memories of gamebooks. When I reread the Cave of Time, I felt like I was reading a map made of pages since there wasn't a plot per se. It was pure exploration, which was kind of neat.
However, i cannot consider them GM-free system. The book is the game master. Or, if you want to get pedantic, the author and editors combined forces to become the game master.
In fact, due to the extreme limits to your choices, I would consider gamebooks to be more game master than most game masters!
I do understand that the Fabled Lands series that came back in the 90s tried to create a sandbox environment in a gamebook format. I've picked up the first book so I can take a look.