Gatling and backfire in Bang!
Yes.

From the rulebook:


  Even though the Gatling shoots a BANG! to all the other players, it is not considered a BANG! card. 


(Emphasis mine).

And the Wikipedia for Bang! says:


  BackFire – Counts as Missed!. Any player who targeted you with Bang! is now a target of Bang!.


When you play Gatling, you target every other player with a Bang!. So BackFire will respond to Gatling just like it would to a regular Bang! card.