Update deployment and maintenance will be taking place from until - Game Servers will be unavailable during this time and other services such as the website may not be available.
This guide only covers installing and mounting Counter-Strike: Global Offensive using Lambda Game Utilities. If you wish to do this process manually, you can find plenty of tutorials on the Internet on how to do this.
It seems that mounting via Steam, even if you have the CS:GO legacy branch enabled, is not working. This is the only method that consistently works.
If you have not already, install Lambda Game Utilities: https://lambda.tf/docs/lambda/content-mounting/game-utils