How to unlock gungeoneers

I've seen on a wiki that there is a ghost gungeoneer and a robot gungeoneer but it didn't say how to unlock them, so how do I unlock more gungeoneers ?

Solution 1:

To unlock The Robot, you have to unlock the first shortcut that takes you to the Gungeon Proper (2nd level). When the Tailor moves on to the next floor, you can go back into the elevator shaft room on the second level and find a busted television, an active item that is thrown when used and dropped when you dodge roll.

If you take the busted television all the way to the 5th level, you can give the item to the Blacksmith to unlock The Robot.

Wiki Link for The Robot

Solution 2:

I only know how to unlock the bullet.

after at least have defeated 1 past, some bullet kin (the standar enemy) will be replaced by a bullet kin with red capes. if you stay alone with him and you don't kil him, he will dissapear. after these occurs five times, you will unlock the bullet.