There are a few fixes if you experience stuttering in Grand Theft Auto V. First, in the Nvidia Control Panel under Manage 3D settings, go to the "Program Settings" tab and chose Grand Theft Auto V from the dropdown menu, and set the following settings:

Maximum pre-rendered frames: 1
Multi-display/mixed GPU acceleration: Single display performance mode
Shader Cache: Off
Threaded optimization: On

There is also an issue with Grand Theft Auto V where the game is not setting itself to High Priority when it launches, and instead the launcher sets to High Priority. To fix this, open the task manager and set the priority for GTA5.exe to High, and gtavlauncher.exe to Idle. This should fix any stuttering you encounter with the game.

Alternatively you can create a batch file to launch the game with these settings. Instructions for creating this batch file can be found on the Steam forums: steamcommunity.com/sharedfiles/filedetails/