While a lot of the below handwaves over a lot of time they'd need to spend learning by experimentation and failure, if they have the patience, know-how, free-time, and resources to smelt steel (or even melt salvage) they can figure out firearms. They can do black powder bolt-action rifles and revolvers for sure. If they've reached late-19th century tech they can do modern-ish smokeless powder in submachine guns and assault rifles.
First a little bit about "gunpowder".
"Gunpowder" is technically "black powder", a combination of sulfur, charcoal, and potassium nitrate. It detonates relatively slowly requiring long barrels to build up speed, produces a lot of smoke, and leaves a sooty residue that builds up and fouls the weapon.
If they can figure out steel, they can figure out black powder. It's fairly easy to make, and has been made for 1000 years. Sulfur and charcoal are easy to get. Potassium nitrate requires a bit of chemical know how, but not much.
With black powder you can have revolvers, shotguns, rifles, and artillery. You can have simple repeating firearms, bolt-action or lever-action rifles. These don't require any special materials or high quality springs, something surprisingly difficult to make. Since black powder is so low pressure, they don't even require steel, they can be made of iron or brass. Besides, anything more sophisticated will get fouled with black powder residue and fail. No machine guns. No semi-automatic pistols like a Colt-45. No self-loading rifles like an M1 Garand.
You can have normal metallic cartridges, making brass casings is easier than making steel and they can be reused. You can have detachable magazines or stripper clips. However black powder bullets will be slower, and so less accurate and shorter ranged. The lower velocity means bullets need to be heavier to have the same impact. Instead of .30 caliber, you'd be firing .45 or .50 caliber like .45-70 Government.
Smoothbore barrels are easy, anyone with a lathe can bore out a polished steel tube. Rifled barrels require some know-how but are not hard, they've been around since the 15th century, but the quality and accuracy will vary wildly.
You might wind up with something like the 1903 Air Service rifle. A bolt action rifle firing metallic cartridges with a detachable box magazine. Except in black powder, so the barrel would be longer, and firing larger caliber bullets like .45-70 Government. And the parts will be of a lower quality. Watch how Karl of InrangeTV smacks around the bolt of a Mosin-Nagant, it'll be like that but worse.
You can have explosives, but not high explosives. "High" meaning "high velocity" meaning the detonation velocity exceededs the speed of sound creating a shockwave. Black powder explosives are good for clearing stumps, less good as an anti-personnel weapon.
What everyone thinks of as "gunpowder" used in modern firearms is actually "smokeless powder", one of various different compounds that detonate at a high velocity and leave little smoke nor residue. This allows for shorter barrels and more complex mechanisms.
The first really successful smokeless powder was nitrocellulose or "guncutton" in the mid-19th century. This is nitric acid and wood fiber. Easy to make by anyone with some chemical know how. Quality will vary while they gain experience, and there are various improvements to be made.
Having smokeless powder opens up the whole range of modern firearms. The higher detonation velocity means shorter barrels, smaller calibers, higher velocities, longer ranges. The lack of residue means automatic firearms are now possible.
Oddly enough, some modern firearms are easier to make then old time ones. Many modern firearms have simplified construction requiring mostly stamped sheet-metal and very little careful work. You do need good spring steel for springs. Submachine guns are particularly simple, you can be cranking out Sten guns. These were so simple, the were made by resistance forces in occupied Europe. Here's examples made by the Poles in the Warsaw Ghetto.
Later you can make milled AK-style assault rifles, and later cheaper stamped assault rifles. Milling is expensive and requires special tools. Stamping is cheap, but gaining the expertise necessary to do it well enough for an automatic rifle is more difficult than milling as was experienced in the development of the AK-47.
Other simplified emergency and "last ditch" designs from WWII like the Gerat 06 would be possible. Quality will be an issue, and expect many jams while your gunsmiths gain experience.
Smokeless powder also leads to nitroglycerin, the high explosive content of dynamite. High explosive means more effective artillery and grenades.
Lubricants and Solvents
"Gun oil" is not necessary to fire a gun, but it is very helpful to prevent corrosion and wear from metal parts rubbing. Most of it is mineral oil (petroleum based) but vegetable oils work fine, too. The important thing that it's well refined, not break down at high temperatures, nor gum up at low temperatures. The lower the quality the more often you'll have to clean it off and reapply.
Solvents for cleaning are much simpler. Alcohol will do and that's very easy to make. Again, it's the level of refinement that matters.
These are a problem. Anyone can make a smoothbore barrel, but without a rifling twist they will be very inaccurate. This may be fine if you're filling a room with lead with your shop-made Sten gun, but if you want to hit anything at any sort of range you'll need rifling.
Rifled barrels are difficult to make without specialized equipment. Fortunately if you're in one of the countries of the world with a large number of guns per capita, like the US with 1 gun per person (that's 300 million guns), you'll be able to salvage rifled barrels from any number of otherwise non-functional guns. It's easy to take a rifled barrel from another gun and cut it to the desired length to fit, so long as it's the right caliber.