Several of the characters in the game have special rules that allow them to do things when they are damaged. For example, one character, when "damaged by another player" may then draw a card from their hand. Do these abilities trigger only when directly damaged by another player (i.e., Bang!) or do they also trigger off of effects like Duel! (which deal damage, but lack the Bang! icon) or even effects like Indians (wherein every player who does not discard a card loses a bullet)?
The answer given is correct. If you consult the rulebook in the Bullet, which gives great explications of the character abilities, it says the following about El Gringo: "each time he loses a life point due to a card played by another player, he draws a random card from the hands of that player (one card for each life point). If that player has no more cards, too bad!, he does not draw. Note that Dynamite damages are not caused by any player." Thus, any card played by another player that damages El Gringo, besides Dynamite, enables El Gringo to take a card from the attacker's hand.
The same goes for Bart Cassidy, who for "each time he loses a life point, he immediately draws a card from the deck." Thus, he draws a card for when he is hit by Indians!, BANG!, Gatling, etc. Since this character ability's activation is not specific to cards being caused by other players, if Bart Cassidy has Dynamite explode on him (and he survives), he draws 3 cards from the deck.
If you are interesting in BANG! modding, custom expansions, strategy, variants, card production, and FAQS, check out The BANG! Blog (http://bangcardgame.blogspot.com). TB!B released Death Mesa, and is also working to release Robber's Roost, both unofficial expansions to BANG!.
In the copy of the rules I've consulted, El Gringo's rules text begins:
Each time he loses a life point due to a card played by another player...
I think it's pretty clear from that that, as long as a card was played that can be held responsible for damaging him, El Gringo's special power will trigger. Whether or not the damage was "direct".