libgpm: delete (#19622)
libgpm: delete (#19622)

- This is a Linux-only formula. It's had 2 installs in the last 365
  days.
- I checked Homebrew/linuxbrew-core, BrewSci/homebrew-bio and
  Linuxbrew/homebrew-xorg taps and nothing `depends_on` it.
maxim-belkin/homebrew-core,rwhogg/homebrew-core,rwhogg/homebrew-core,Linuxbrew/homebrew-core,Linuxbrew/homebrew-core,maxim-belkin/homebrew-core