Translation:You can have this book for free.
Possibly because then you are treating "gratuitement" as an adjective rather than as an adverb (the "ment" ending tips us off that this is an adverb. It's true an adjective would also come after the noun it modifies in French) It's a granular change of meaning - "this free book" refers to giving a book which is normally free; "this book for free" refers to giving away a book which normally costs some amount.
It's my opinion that "for free" can be shortened to "free." As a native English speaker, living in America, I would say, "You can have this book free" and it would make me think someone was not an American English speaker. Working "for free" might be a more common way to say that rather than "working free" that might imply other meanings like at liberty or something.
I am a native English speaker and I tutor English A lady in her eighties from church told me that in English get is the ugliest word and that in her time people rarely used it. She was a school teacher. Please, avoid get. There are so many other words to use.
I am constantly being told by a local English teacher that we should not say "for free". Something is either "free" or it is "for nothing" (or presumably "gratis"). I admit to using the expression (hence being corrected), and I pass on the comments for what they are worth! Edit I note there has been quite a bit of discussion about this, but I reiterate the point "for free" in whatever context is wrong. It is in common usage, and so will no doubt become accepted (and is currently used on the B.B.C.), but to be grammatical, one should say free,free of charge or for nothing,
I am a university graduate and I live in England and I am happy to say 'You can have this book for free' is excellent English. It means free of charge. To say 'You can have this book free' suggests to me a desire to unshackle it from its chains. You would never say 'This is for free' that would make me cringe. 'This is free' and 'you can have it for free' are both good English (or you can pay if you like). Don't say 'This is for free', or 'you can have it free' they are both wrong.