Autogun charges are perfect for killing gremlins and are quite effective at killing beasts but you need to wait for them to start attacking, dodge, and the attack while they are stuck performing the attack.

I can't talk about the Valiance but if it can stop turrets like some people seem to say, then it would be a great asset to a pure gunner set for the sake of turret demolition and knockback.

Polaris is a brute of a gun but has little place in a gunner set. On sets that I do use the Polaris, I use it to purposefully bounce enemies back and forth between two shots while running at them with a sword giving them no time to attack me. Other uses for it include turrets and panic mode when you must get everything to get off of you but as a gunner who likely has room to breathe, the Valiance could do that much much better from what I've seen.

I personally prefer the Catalyzer but that really comes down to enjoying the playstyle of it. Biohazard is firstly great because I feel at least one poison gun in a gunner set is a necessity due to how fast menders can heal. More importantly though, the charge attack explosions do insane damage and imo, make RJP a very very fun place. I should also mention that the Catalyzers are one of the most team based weapons due to the ability to stack charges on the charges of other players and being able to set them off regardless of who placed them. As for the Shadowtech, it's probably a better choice if you're more proficient at alchemers plus it seems you already have one. If you can't decide, get both.

As for Antiguas, they're pretty meh. The damage they do in comparison to the potential damage of alchemers is pretty sad along with how slow they make you trying to do the same damage but the range and bullet speed can make it a useful gun to use until you can switch to something better after approaching. The charge attack is the best part of the antiguas due to being able to shoot through crowds and it basically activates an autogun mode that can be quite risky if you don't know how long it will make you stand still.

Autogun:
Most people just use the charge in close range. It's really more of a turret/shotgun than anything. You'll spend most of your time charging and counterattacking.

Magnus:
It's fun, especially if you learn how to switch-shoot, but its DPS is terribad in comparison with other guns :(
They do interrupt devilites and wolvers though.
Iron Slug does exactly the same damage as Callahan vs neutral monsters, so the only reason to even want to try Iron Slug is for the novelty of being able to interrupt slimes or turrets with regular shots (and if you're in a post-basil depth, two quick shots in succession).

Pulsar vs Voltech Alchemer:
Voltech-->Storm Driver is more versatile - It works great in close range, and works exactly the same in long range. It's outdamaged by Nova Driver and long range Polaris. Voltech also works as a decent devilite fighter because you can shock them into not being able to move.

Blaster vs Pulsar:
Valiance is better as a 1v1 weapon than Supernova. It does everything ... averagely, and isn't bad for anything, while Pulsar lines are better for turrets, hitting small groups, and soloing safely due to its long range knockback.

Catalyzer vs Shadowtech:
Shadowtech-->Umbra driver is more versatile due to its speed and predictable strength. You said you already have alchemers, so I'm sure you know how to use them. I'll talk about catalyzers since less people use them.

Catalyzers needs patience to use right - whether you have a Vortex bomber pulling in monsters together for a good explosion, or waiting for monsters to group together before you detonate charges. It's a lot more fun if you have other catalyzers in your party, and nearly unusable if you're in a party full of swordsmen, because they will either kill or knock away monsters. Its regular shots are decent damage and great fire rate, but slow-moving, so it's better if monsters are chasing you, and pretty much unusable if they're chasing others. Biohazard works well in RJP because monsters are so close together. Catalyzer-->Neutralizer works on zombies in addition. My gunner-vortex loadout for FSC brings Neutralizer in place of a sword, for larger/tighter groups of monsters.

Since you said you have alchemers already, if you're going pure guns and want to use catalyzers, start with an Alchemer out since the ricochets are useful for getting them to chase you, then you can switch to Catalyzer and start laying down charges. In close range, you can bait an enemy to begin an attack, then release charge. The shooting animation pushes you back and possibly out of their damage range, if you space right.

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Also, every gun reliably can interrupt/knock down non-resistant monsters during their windup by using their charge attacks. Catalyzers can interrupt turrets reliably because they're friendly to charge attacks. Alchemers, valiance can also use charge attack to interrupt them too.

Only a few guns can interrupt with their regular shots though.

Valiance works magic against Greavers. It interrupts theirs attacks nicely and knock them far enough to not get you caught into haze cloud (if they actually have a chance to produce one). Theoretically Greavers, as all fiends, are vulnerable to piercing, but I wouldn't advice Callahan or Autogun against Greavers. Fast back-pedalling and keeping enemy at distance is a key in case of Greavers and that's what Valiance is best for. Pulsar won't do the trick because Greavers quickly fly to a distance too short for a Pulsar to produce knock-back explosion.

Polaris is good against Devilites. I suppose same goes for Supernova, but I don't have first hand experience with Supernova.
Personally, when entering fiendish level solo, I always equip Valiance and Polaris.

Magnus is efficient against Wolvers and Gremlins if you have problem with their dodges. Autogun is probably more deadly against Wolvers but somewhat more tricky to use so I'd go with Magnus, especially if you like kiting, which is still doable with heavy handguns.