Where to download Grand Theft Auto V legally for free?

You should have mentioned the platform which you are searching the game for. Are you looking to play this game on PC, Playstation, Xbox or some other platform.
This is not a free game for all the above mentioned platforms. You can play it for free only on Android or iOS smartphones and tablets. Even on these platforms, unless you make certain in-app purchases, you won't be able to proceed much further in the game. This is a limitation to all good games that are published by big companies.

You can't download this game legally for free. Even if it was available for download (for PC for example) you would need to download around 6-8 GB of data for the entire game with all files. Some high-end games like these usually sell in a set of 2-3 DVDs.

No, its not possible to download GTA V for free. I searched on the internet a lot but GTA V is not available free of cost at the moment. If you truly want to play the game then buy or ask your friends who have it. This is the only option for you right now.

I think you got incorrect information. Currently GTA 5 is released only for Xbox and Playstation. It is scheduled to be released in January 2015 for PC. So there is no chance to download it for PC yet. You will have to wait until next year.