Snake bitten stranger

At some point in chapter 2 when roaming around the world, you come across a stranger who is bitten by a snake and is poisoned. I was too late to give him the cure, and the man died.

Does this mean I wont be able to complete his dialog

and get the 'free' gun in this playthrough?

Yes. These are randomly spawned encounters though, so don't worry too much about it :)

NPCs like this will appear as White Dots on your map, or White Question Marks and will call out loudly - if you can hear an NPC hollerin' from a ways out, particularly outside a town, odds are good it's a ? that's available.

I have had multiple people bitten by snakes, the 1st got me

a free gun

and the 2nd got me

one free item from the general store.

There may be more but I am unsure if once they're dead you miss out.