In Episode 123 of the manga is revealed,
Pig god can't be hurt by firearms
We know other S Class heroes can't be hurt by firearms, such as Zombie man. But we see an S class rival as Garou in his first stage, someone who is capable to defeat the weakest S class heroes, having to divert a large amount of bullets from a gatling gun and having his hands bleeding from that. Is it known which S Class heroes can't be hurt by firearms, and if there is any who can be damaged by them?
The one hero that comes to mind is King. As mentioned here, King is actually weak. He has no special abilities whatsoever, aside from being a video game expert.
It is no overstatement to say that he is the weakest among all the heroes in the Hero Association. He's just a normal person and like any normal person, getting shot by firearms would severely hurt or incapacitate him, depending on the type or where he would be hit.
As mentioned by another user, the answer to your question depends on the angle of the question. Do you wish to know whether, for example, a given S-Class hero would be able to withstand a gun-shot at point-blank range, or if they could conceivably deflect or endure firearms in some way ? I'd lean towards the latest definition, since you mentioned Zombieman ; and Zombieman most definitely is shredded by gun-fire, even though he can easily regenerate. Let's look at all known S-Class heroes :
17 - Puri Puri Prisoner : As the weakest S-Class hero, Puri Puri Prisoner probably is vulnerable to firearms, even though he possesses superhuman durability. However, he was visibly hurt by Freehugger's spikes (Chapter 76), and I wouldn't put Freehugger's damage output above a firearm. Puri Puri Prisoner has evolved since, and might be able to withstand bullets with Angel ☆ Bristle Armor or to deflect them with Angel ☆ Pitter-Patter.
16 - Tank Top Master : Although Tank Top Master ... tanked several of Garou's strikes, he ended up pretty badly hurt. I do not think he can withstand gunshots despite his inhuman strength.
15 - Metal Bat : Metal Bat's forte actually is his durability. He fought against Elder Centipede and was thrown into buildings, yet wasn't fazed by the ordeal. His fighting spirit actively reinforces his durability as the fight goes on. Moreover, he can move and fight just fine even while bleeding profusely and having several broken bones. Even if he's vulnerable to gunfire, depending on the wound, Metal Bat can probably endure the pain and continue fighting.
14 - Genos : Genos is made of literal metal. He's most likely fine against gunfire, though bigger calibers (anti-tank shells for example) may pierce his armor.
13 - Flashy Flash : Flashy Flash is very durable, being able to fight 2 Dragon-level monsters at once without taking significant damage. However, his best asset is his speed ; and he clearly goes much faster than bullets. In the manga, we saw him complete a whole fight before a destroyed bridge had time to crumble (Chapters 95 - 96). In the webcomic, Flashy Flash can also perform an offensive combination in 0.01 seconds. So this fine ninja would just avoid gunfire altogether.
12 - Watchdog Man : Undefeated by monsters and able to utterly crush Garou while toying with him, Watchdog Man boasts impressive speed. However, since we know very little about this furry hero, we do not know if he would fare well against bullets. Given his strength level, I would argue he probably has a solution.
11 - Superalloy Darkshine : Described as having the "most resilient defense in the hero world", Superalloy Darkshine probably is barely tickled by bullets. Destrochloridium's drills, described as "strong as steel", broke upon striking Superalloy Darkshine. The hero can also shimmer, a mysterious ability which lets him reflect incoming attacks outright.
10 - Pig God : As you described yourself, Pig God is impervious to gunfire. He simply absorbs bullets within his fat without withstanding damage. He also has an accelerated healing factor, meaning any damage would be quickly resorbed.
9 - Drive Knight : Just like Genos, Drive Knight is a cyborg -- a more advanced one, at that. We do not know his upper limit but Drive Knight should be fine against the majority of gun-fire.
8 - Zombieman : Zombieman gets absolutely shredded by regular gunfire, but can easily regenerate the damage.
7 - King : King is even less resilient than a regular human. King dies instantly against gunfire.
6 - Metal Knight : His various robots are all armored, and much like Drive Knight and Genos, can most likely withstand most firearms. His human form, however, goes down like anybody else.
5 - Child Emperor : Being a regular child (albeit able to shield himself against heavy blows), Child Emperor probably fares badly against all firearms. However, his many gadgets and seemingly-automated defense systems most likely have built-in countermeasures against firearms.
4 - Atomic Samurai : Kamikaze's physical durability is immense, being able to withstand a barrage from the almighty Black Sperm. However, much like Flashy Flash, his main asset is his ludicrous speed ; Atomic Samurai can move faster than other characters can perceive and shred them into miniscule particles in the blink of an eye. He would certainly slash down incoming bullets.
3 - Bang : Bang has been shown to be even faster than Atomic Samurai, outrunning him while escaping Boros' crashing ship. He should thus easily be able to deflect or avoid bullets too.
2 - Tatsumaki : As the most powerful known S-Class hero to date, Tatsumaki boasts catastrophically power psychic powers. Most notably, she is perpetually surrounded by a psychic shield protecting her from every conceivable form of harm. That forcefield has been shown to withstand gravity increased 300 times, and a full-power heat blast from Orochi, King of Monsters. In both those cases, Tatsumaki suffered virtually no damage. Bullets are child's play for her.
So here's a full breakdown ! Since S-Class heroes have been never shown facing firearms, we can only guess how they would respond to such a threat.
