How to Use the Downloaded Game Files?
You need the Steamtools program to import the Manifest/LUA files to the Steam for the games to appear in library.
Download the Steamtools from here.
After installing, follow the steps shown in the video below.
How to Launch Grand Theft Auto Games?
To play Grand Theft Auto games offline, you need the Bypass Launcher, which skips the Rockstar Club and other unnecessary anti-cheat tools.
Download the Nightlight Game Launcher from here. And continue watching the tutorial video below.
For Grand Theft Auto V make sure to put -nobattleye in the Steam Launch option.
What is Process with Manifest?
The Manifest file contains essential data that allows the Steam client to recognize and install the game properly. When you use the Steamtools program, it helps import the Manifest/LUA files, making the game appear in your Steam library.
If your game is not found in the normal Download button. Give this option a try.