A gamebook is an amalgamation of two things - a game and a (story) book. It's all in the name, really.
What this means is that you canget gamebooks that are more game than book and gamebooks that are more book than game. I'm going to define the game and the book part of gamebooks, state what characteristics each one has and what type of player would like them.
I will refer gamebooks that are mainly game as GAMEbooks and gamebooks that are mainly book i.e. they focus on story and character rather than stats and winning as gameBOOKS.
Here are some characteristics that make a gamebook more game than book.
Clear objective for victory: Since there is more emphasis on game, there is a way to win and a way to lose. The objective is usually something like kill the wizard or find the treasure or escape the dungeon. Sometimes, the objective is revealed to you part way through the gamebook, such as in Creature of Havoc or Black Vein Prophecy but it is always there.
Few (usually one) satisfying ending: Since there is one objective, most GAMEbooks only have one good ending where you meet the objective. Most other endings usually result in your death, or, if they don't (for example, if you decide to spend you life being a monk in the Crimson Tide), the text usually states that you have failed in some way.
More life and death struggle: Since in GAMEbooks, you either win or die, there are plenty of combats and life threatening situations for you to fall into.
Very little backstory: GAMEbooks are moer likey to be the kind of gamebook with a few paragraphs about some evil wizard which aren't really relevant to the rest of the game as you then just have to crawl around a dungeon to kill them. Warlock of Firetop Mountain is a classic example.
Use of stats: There are more stats in a GAMEbook than in a gameBOOK. These stats usually revolve around the abilities of the character rather than anything to do with the story, such as their fighting and magical abilities. Characters in GAMEbooks also have some value to measure hit points as opposed to gameBOOKs, where, if death is a possibility, it is usually due to the text rather than any situation.
Choices mainly revolve around resource management rather than what you would do as a character: Since you have a load of stats, the rules usually have a way of manipulating those stats. Destiny Quest is the perfect example of this. Should I use my charm ability now or later? Which die shall I reroll? Should I drink this potion of vanish? Should I take the speed enhancing item or the brawn enhancing item? These are all very interesting decisions but none of them revolve around the story.
Your character and the NPCs are mainly defined by their stats and equipment rather than by their interactions with others and their decisions: A lot of NPCs won't even have names as they are there to provide you with a combat or some other stat based challenge. Your character will probably be defined by which stat is highest or which skills and spells they have (chosen based on their ability to win you the game rather than flavour. It is not uncommon to pick a wizard with both healing spells and damage dealing spells.)
Examples of GAMEbooks: Fighting Fantasy (with Legend of Zagor taking an extreme approach), Destiny Quest, Trial of the Battle God, some Tunnels and Trolls solos.