From the game's encyclopedia
Handgunners are Men-at-Arms who are armed with potent gunpowder weapons.
While each kind of Handgunner is unique, they generally Counter Heavy Infantry, and are in turn Countered by Heavy Cavalry and Light Cavalry.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Handgunner is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Handgunners are Men-at-Arms who are armed with potent gunpowder weapons." It is part of Men-at-Arms. Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.