This is not an Addon nor a replace.

You need to Install:

Vehicle Builder: https://www.gta5-mods.com/scripts/vehicle-builder-make-vehicles-in-game#description_tab

Script Hook Dot Net Ehanced - https://github.com/Chiheb-Bacha/ScriptHookVDotNetEnhanced/releases/tag/v1.1.0.4

LemonUI - https://www.gta5-mods.com/tools/lemonui

Place the "VEHICLE BUILDS" folder inside the "SCRIPTS" folder.

In the game, press F11 to open the menu and navigate to "MANAGE YOUR BUILDS". Load the vehicle you downloaded.

To use the sniper, just put away your weapons and press right mouse to aim.