If it is a "pre-installed" version, simply locate GTAV.exe or a custom Launcher.exe in the main folder to start the game.
Always scan files from third-party sources with updated security software. Site-specific archives like those from AGFY are popular, but the integrity depends on the specific mirror used. AGFY-Grand Theft Auto V.rar
All necessary game data files ( .rpf , binaries, etc.). If it is a "pre-installed" version, simply locate GTAV