1. Right click on csgo in your steam library

2. Click on properties

3. Click on the "betas" tab

4. Click on the scrollable tab

5. Select the demo_viewer version

6. Wait until the end of the download

7. To play the game with this version you have to launch the game from it's root folder inside "steamapps" > "common" > Counter-Strike: Global Offensive

8. You can run the game with launch commands (example : -tickrate 128) by creating a shortcut, going to properties of the shortcut and adding your launch command after the "target"

9. And just right click on csgo.exe launch it as Admin.

That's it ! enjoy your sick 64 ticks (or weird 128ticks) gunrange runs.