Gunner is an enemy listed in the Enemies bestiary table.
Nothing else about it is recorded anywhere on this wiki: no floor, no health, no damage, no drops and no behaviour. It is listed next to the Goblin, the Camel, the Pot Mimic and the Puffer Man in the table, but position in that table does not reliably indicate which floor an enemy belongs to.
This is a stub. The Enemies table lists this enemy but has never carried a description, and its table image is a placeholder borrowed from another enemy. Nothing about it was found in the Steam guides, forum threads or the official manual read for this wiki, so everything above is all that is currently recorded. If you meet one in game, this page is worth filling in.