The only kind of GTA 5 download out there is beta access. This is a very selective process that very few people are allowed to enroll in.
There are a few websites out there that claim they have a way to get beta access, but enter these sites at your own risk. The chances of any of these sites actually having any kind of GTA download is slim to none; so again, enter any of these sites at your own risk.
Realistically there is no way to download GTA 5 for free. The only way to really play GTA 5 is to wait for the official release of the game. Until then everyone’s going to have to wait patiently and watch the trailers and gameplay footage in anticipation of the official release.
Check out the "suggested by the author" links below for more GTA 5 information.