How do you earn 'Gnome Alone' (the honorary achievement for Gnome Chompski)?

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?

• Launch the game
• Start a campaign (singleplayer is fine)
• Earn the achievement:

I earned it on Dead Center, but I assume any campaign will work.