Man pages matching name gfx_mode_select_ex in section 3alleg4:

gfx_mode_select_ex.3alleg4.en	debian 10 / allegro4-doc
gfx_mode_select_ex.3alleg4.en	debian 11 / allegro4-doc
gfx_mode_select_ex.3alleg4.en	debian 12 / allegro4-doc