As others mentioned, it is an achievement from Half Life episode 2. Player has to carry a garden gnome to the end of the game. Which is not too bad, minus the extensive driving sections (sometimes under fire) where the gnome is bouncing around the vehicle.

It is a fun feather in your cap, but definitely not suitable for a first run through of the game.

I assume this is the relevance:

https://left4dead.fandom.com/wiki/Gnome_Chompski