From this comment from this 2009 doomworld forums thread:

It was an in-joke, and don’t feel bad for not understanding it completely… I don’t really get it either :P

If you were wondering just in case of writing a story or some such, I doubt anyone would care if you re-define what the letters AGM mean.

Someone has then proposed a back story in 2018,

In 2017, an issue has been open on GitHub, and a related freedoom forum thread was started with a poll.

https://www.doomworld.com/forum/topic/140613-what-does-agm-mean/