Over at Camp Gulf, if I'm killing all the Rangers, will they respawn back?

Maybe not right away, but if you leave and go back later, there might be more ready to avenge their fallen.

Most non named npc's do, outside of a quest resolution for them, so I would guess yes as long as it was not a specific quest requirement.

Most free roaming/non-named respawn in about 3 game days, from my observation. But there are some groups in specific areas/buildings that will not respawn at all. I think the Powder Gangers in the jail may be one of them (?), and other ones. So you could try saving, killing them all, "wait" for three or four game days and see if they come back. Assuming you actually want them to.

What a nice way of putting the fact that they respawn. <_<

Short answer: NPCs respawn (if they are enabled to do so, IE not a quest object) when their cell respawns.

In FO:NV the cell respawn timer seems to be three days but sometimes due to the ineffable machinations of the engine a cell will skip a cycle and it'll take 6.

Not 100% sure but powder gangers seems to respawn ...unless In such case gangers stop to respawn also at various camps around ...or so it seems to me.

NPC's don't respawn indoors IIRC.
So one way to clear out a place could be to lure everyone inside a building and kill them there.

It's not an inherent feature of the engine that NPCs don't respawn indoors, in fact in quite a few places in FO3 they did (leading to the famous skill book farming exploit).

I'm not actually sure which FO:NV areas are set to respawn and which are not.

Most quest locations (IE Bison Steve's, the vaults, ect) do not I can confirm.

Some of the NPCs seem to respawn even quicker than that. There seems to be a fresh supply of Freeside Thugs every time I travel through there (there is now quite a pile of bodies in parts of Freeside as they seem to spawn quicker than the engine can clean them up), same with the Fiends in the Fiend area - the NCR soldiers guarding that area also seem to respawn quickly.

While were on the subject: Do Caravan Merchants and their entourage respawn?

It is possible that some cells could be set to respawn faster than the generic timer, I suspect that Quarry Junction might be this way because my attempts to "thin the herd" then go sleep off my svcking chest wound and come back bright eyed and bushy tailed and full of vinegar and .50cal incendiary ammo always fail miserably, they seem to come back or move around or something because there is invariably a pair of deathclaws sitting on the northern hill.

Oh and the freeside thugs are spawned dynamically via a scripted ambush I'm FAIRLY certain, because at night they seem to occur with at least triple the frequency.

Do any of the Legion camps/bases ever respawn? I'd love to be able to go hunting with Boone every couple of nights, but I've never seen it happen...