As Frayling said in the FAFDM dvd commentary, the thing with the leather gauntlet somehow didn't come across as clearly as the script meant it to:
Manco ("one armed") is a bounty hunter who only uses his left hand in all his non-shooting activities, so that his right (ie. shooting) hand is free for whenever he needs to draw. On that right hand, he wears a trademark leather gauntlet, which he is known for. That's why when he joins Indio's gang, he is not wearing the gauntlet -- so that no one will know who he is. And just before he shoots the 3 gang members atop the mountain, he is wearing the gauntlet again, letting them know "I am Manco. And you are dead!" and when Indio catches him sneaking out of the area where the safe with the money was, he has the gauntlet and says "Put it on," ie. "I know who you are!"
So as Frayling said, this thing with the gauntlet perhaps didn't come through in the movie as clearly as it was meant to (and indeed, I did not know all this till I heard Frayling's commentary. But once I heard it, it all makes perfect sense