The official Left 4 Dead Blog has stated in a 2020-11-18 blog post:
An update has been released for Left 4 Dead 2.
- Honorary achievement for Gnome Chompski's upcoming journey into space on Rocket Lab's "Return To Sender" mission.
Update: At the time of this edit, Gnome Chompski has already been launched into space. However, I've opened the game but it shows that the achievement is still locked.
Gnome Alone achievement:
How do you get this achievement?