To dry-gulch is simply to "ambush."
There are no connotations regarding motives. I suspect guns are nearly always involved though.
and the rocks it pummels.

• James Berardinelli

sounds painful.
Where there's smoke, there's barbecue!

It's a term to describe an ambush robber, someone who lies in wait and then (typically) murders their target unseen and then loots the corpse.

you might remember also that Ben Johnson uses the term in "The Wild Bunch"
"You won't get a chance to dry-gulch me old man"

Wrong sir.

Which is certainly true of Brando's character- shooting from a distance, never getting close- and his trademark seems to be getting people at very undignified moments (on the toilet, having sex).

I don't think it could be reserved for someone who could be legitimized so to speak as a sniper, that implies perhaps in some way that he would be employed gainfully and repeatedly in that function.
My two cents worth are, I think it came to mean anyone who would shoot you from hiding, certainly, but also would stoop to shooting you in the back. So a dry gulcher would be a craven, base coward with no sand in his craw at all.

Thank you very much, that makes sense.
"You think I was telling you the truth? Maybemaybe."

Yes, it's slang for a hired killer, but one who specializes in ambushing from a distance, as in long-range sniper firing - a shooter hidden out of sight of his victims. (I'm guessing the name came from the shooter using a dry gulch to hide in, so his victims couldn't easily see where the shots were being fired from).