I'm not a native speaker, but I think you're correct. Gercekten gives the sense of "in truth/reality" as if the speaker was confirming what either they or others had suspected, i.e., the book truly IS difficult. "Bu kitap cok zor" is closer to "This book is really/very hard".
"Bu kitap gerçekten zor." Translation: This book is really hard.
Why not ..this book is indeed hard ?
I can see what you mean clearly & Duo did not accept your answer for this reason.
Really (adverb) - in actual fact.
Indeed (adverb) - really or certainly. Used to emphasise something that is already known. In this example (zor) - hard.
"Bu kitap gerçekten zor." Translation: This book is indeed hard. - Correct.
I like your answer & Duo may not so a like from me ^
This is a very misleading sentence. 'This book is really hard' is not how an English speaker confirms that a book is in fact hard, which is what I think the Turkish sentence means. As a native English speaker I would say 'this book really is hard' or 'this book is definitely/actually hard'. But 'this book is really hard' means exactly the same as 'this book is very hard'.
I absolutely agree and would also say 'really is' instead of 'is really' to convey this meaning. The way 'really' was used, I'd understood gerçekten as an exact synonym for çok. Perhaps 'certainly' (suggested by someone above) would be better for getting across the Turkish meaning, assuming I do now understand it correctly!
"gerçekten" translates as "really" because it also means "certainly /actually". This is slightly different than "çok" which means "very" or "extremely".
So although we can often use "very" and "really" interchangeably in English, I think this course separates them so that we can learn the difference between "gerçekten" and "çok". :-)
There are situations in English, as well, when we can't swap them... For example: "You really are my friend." cannot be "You very are my friend."
quite = oldukça ; gerçekten = really /actually /genuinely
I can't speak for everyone, but in my dialect of English, "quite" is a middle-ground between "a bit" and "very". While "actually /really" is more certain-- as in, there is no debate or no middle ground, it's really a difficult book.