To download and play on PC, you must purchase the full game, which has included the Online mode since its original PC release on April 14, 2015 . While "GTA Online" is often discussed separately, it is technically an integrated mode within the main GTA 5 game files for PC. Official Download Platforms
You can legally purchase and download the game through several official digital stores. Each requires you to install their specific launcher and create a Rockstar Social Club account to verify your license. How to Download GTA 5 on PC & Laptop - Install GTA V Download GTA 5 Online (GTA 5 Online) PC Game 2015