Essentially, gear is mostly the same as you would have it, although the system might be a tad complex. For the most part, the guns available are the guns that the City salvaged from the world it took over, things ranging from a simple Smith and Wesson pistol to a Heckler and Koch G11 to probably a dozen types of automatic weapons, though for the sake of brevity I will simply make it as such. Your gear is limited to four categories and each of those categories into a few different subcategories.

For instance, for armaments and equipment, you have weapons and ammunition, your armaments or clothes, and any other equipment you may have. For weapons and ammunition, for brevity's sake, it will just be split up into knives, blunt melee, knuckles, pistols, short-ranged rifles, long-ranged rifles, shotguns, automatic weapons, and explosives, missiles, and other such incredibly heavy artillery. Anything past long-ranged rifles is likely only to be found in the most dangerous of Caches or otherwise kept in some protected storage unit to use in emergencies or invasion (unless you're part of a Sector that cares not for rules or safety, in which case all the power to you). On top of this, you can of course have modified sights on ranged weapons as well as other modifications, and given the City's eldritch properties and the fact that some of the many creatures it boasts include Towerspawn, essentially golems made of intricate and unorthodox clockwork mechanisms and what seems like pieces of the City itself, as well as a number of other creatures that affect things around them, it's quite possible to end up with a pistol or rifle that does some crazy things. Exactly what it does depends on what goes into the weapon and what the properties are of the aforementioned unnatural modifier. Other than that, you have your normal attire, although stab-proof or bullet-proof attire is much harder to come by though you usually find at least one in almost every Cache. And as for other equipment, that ranges from night vision goggles to other things that could be useful in the City, but not necessarily attire or a weapon.

Now, as for the Afflicted. What an Afflicted is is essentially a human being that has either been exposed to some of the more unnatural parts of the City, a Constant, or something to the point where their physiology and psychology have been affected and morphed into something much more inhuman. For the most part, if a Constant incites this, it's trying to make you into some kind of Servant of it, and usually these Servants have the same general skillsets, but if it's the City itself, it's almost an ability, physiology, and self-control roulette. Obviously you state whether or not it's even a possibility, and you have control of what happens if your character ends up getting Affliction. It's generally irreversible unless you do something crazy like make a deal with a Constant, which is essentially the only way to undo either the Affliction fully or the negative aspects. But as far as the abilities that come with Affliction, it varies, but usually there's at least one or two inhuman aspects that cause the Afflicted person to become a very, very skilled combatant so long as the other changes aren't hindrances.

Now for the most part, there are five main types of creatures that roam the City proper, outside of Constants and their Servants and, of course, the Afflicted. In order from the most populous to the least likely to be seen, these include:
[list]
[*]Shadow Graphers- The aforementioned shadow people with the pinhole eyes and the slit mouth smile, these creatures are beings of habit and order. Some say this is what happens to humans who are absorbed into the City, and others say they're merely the heralds to the City and were never human. All that's known is that these are either simple messengers, or deadly vicious attackers that can fuse into each other to become things that are truly nightmarish.
[*]The Harmony- The Harmony is a strange beast. For the most part, it looks like a fungus or a growth that usually is on the ground or a wall. And it can completely override your perception and make you see and hear things that aren't actually happening. Some people, including all Afflicted or Constant-allied people, are immune to its abilities, but it's a dangerous, dangerous thing, especially if it gets onto a person. Then, they essentially are displaced from reality alongside anyone within a certain distance of them, unless, again someone makes a deal with a Constant.
[*]The Flock- Another hive mind of sorts. These are birds that will appear out of nowhere and almost darken the sky. They can attack as a swarm, going for the eyes or soft flesh, or they can themselves meld into things like a storm or lightning bursts or a veritable tornado. These creatures are vicious and without remorse, and if they are quiet but present it's because something worse is there and they're more than willing to watch the suffering commence.
[*]Towerspawn- Like I said, these are the City's clockwork and dieselpunk golems. Most of the time, these things are completely subservient, mindless, and only given set parameters of human interaction, but for the most part are more on the destructive side. But there are many Towerspawn more than capable of mimicking human interaction, having their own thoughts and personalities, and even leading or misleading people or Retrievers it comes across.
[*]Cacklers- Mostly these are nighttime creatures, so they rarely make an appearance in the day except in dark interiors or in underground systems. They look like normal parts of the City, street lights, cones, empty newspaper vendors, a fire hydrant, anything. They can either communicate (especially in the case of a street light with its three lights), but most of the ones that will attack you will simply cackle. They'll cackle and you won't know where it's coming from until it reverts to its original form, which nobody has ever really lived to see. If they survived the Cacklers at all, they ran like hell until they reached a Dead Zone.
[/list]

These are not by any means the only creatures the City has to offer, these are just the ones that most often show up. Other creatures appear and can, at times, be more prevalent in an area than the normally occurring creatures. 

Thank you for comparing this to S.T.A.L.K.E.R. by the way! That franchise, even moreso than the Fallout or Metro franchises, was the inspiration to sort of make this RP, on top of lovecraftian horror and just the tiniest bit of creepypasta thrown in for extra flavoring.

EDIT: And the answer to what happens in the City is that really, aside from it mostly taking on the form of a City, there is no limit. The street can pull an Inception and turn upwards and even fully upside down to where the street continues up and above your head almost in an impossible U-turn, manholes can lead to the rooftops, you can walk up walls in a section where the gravity is sideways and pray it doesn't change while you're up there. For the most part, the only sections that aren't city-like are the dominions and homes of the Constants in sections in the very center of the City, which vary based on the Constants' varied personalities, representations, and abilities, and even then it might just be because those dominions are the unnatural equivalent of their living spaces. Aside from Dead Zones, which include Sectors and the narrow paths that connect the Sectors, and the domains of the Constants, mostly because the Constants do not want it, the City is in control of itself and aside from its urban aesthetic being generally the same throughout except in these domains, it is absolutely in control of itself and how it functions and operates, and is known to change on a whim.