The following rules are taken from English manual of Italian version of Bang! The Bullet!.
Regarding the card Missed! the interesting part is:
If you are hit by a BANG! you may immediately play a Missed! - even tough it is not your turn! - to cancel the shot. [...]
Regarding the card Gatling! the interesting part is the note:
The symbols show "a BANG!" to "all the other players". Note: even though the Gatling shoots a BANG! to all the other players, it is not considered a BANG! card.
This note makes me wondering if the Gatling can be countered by a Missed! or not. Reading the rules I think that the Gatling cannot be countered but in this case the card is too overpowered.
Note that also other characters and cards are influenced by how we interpret the Gatling card.
e.g. These characters of the base game are influenced:
- Calamity Janet: she could use BANG! as missed
- Jourdonnais: he can draw! when he is target of a BANG!
- Slab the Killer: players trying to cancel his BANG! cards need to play 2 Missed!