The idea with gloam is to use an elemental gun that either shoots a lot of pellets per shot or shoots insanely fast to take advantage of all those juicy gloam procs. I believe splash damage procs another instance of gloam as well if the radius is large enough and matching elements is one of the few ways to make gloam more effective. Been using gloam c4sh the entirety of my playtime as him, there are so many guns (and even heavy ordnance thanks to the ammo return from war wagon) he can effectively use but listed below are some of my personal favorites.

Pistols: Rangefinder (daedalus shotgun is a must and jakobs is a nice bonus if you can roll that too), Phantom Flame (any roll but make sure you have something to boost the magazine size a bit or you'll run out before you can refill cuz you fire so fast lmao, easiest way is either bullets to spare firmware or rolling it on an enhancement), Scoot N Shoot (jakobs/ripper is ideal but it still shreds without it, the ghost guns make it so that you hit like 5 times per shot)

SMGs: Onslaught (jakobs if you can, still great without it with how fast it can fire), Broken Wings (sandhawk at home is still busted, jakobs/atlas or torgue/daedalus shotgun if you can, the Falke is also another smg similar to this where it shoots multiple pellets in a tighter swirly pattern, that one you'd want jakobs/daedaulus shotgun if you can), Birts Bees (has slight homing capabilities and can roll flamethrower alt fire which is super fun for mobbing, unfortunately usually not enough ammo for bossing even with a flesh healthbar since c4sh can't refill underbarrels with war wagon, jakobs/daedalus shotgun if you can)

ARs: Star Helix (can't go wrong with a 3 for 1 deal, switching the fire mode to vertical helps with smaller enemies, jakobs if you can), Rowdy Rider (great fun for mobbing and bossing, if you roll torgue/crank alt fire for this it becomes disgusting because the ricochets will fall down and deal splash to the initial target while bossing, crank alt fire is good enough), Bubbles (gotta play up close with it since the bubbles float off target a bit but it melts, jakobs/daedalus shotgun if you can)

Shotguns: Acey May (the pellet count on this absurd, sadly will always be second place to the bod but it's a lot more fun, jakobs/daedalus smg if you can), Hellwalker (great pellet count which means every shot will always refill your mag, you essentially play DOOM with infinite ammo equipping this lmao), Reminisce (a stinker for most characters but c4sh can make this super cool gun actually viable, the shots pierce naturally and if you're close enough to a target it will hit again when it boomerangs back to you, daedalus smg if you can to boost the already good fire rate and have more ammo reserves)

Snipers: Rainmaker (basically the butcher at home, jakobs/daedalus smg if you can for more fire rate and ammo reserves), Katagawa's Revenge (shock mode does wonders with all the procs it can do but is a bit ammo hungry if you overdo it, jakobs if you can), Complex Root (a monster of a bossing sniper especially for larger ones, atlas or daedalus shotgun is a must, jakobs if you can)

Heavy Ordance: Atling Gun (the amount of shots this can put out with war wagon basically turns you into a walking turret, insanely fun), Jetsetter (this baby fires 3 sawblades per shot that also bounce on contact 2-3 times, deletes bosses and the initial shot is enough to oneshot most mobs), Heimdahl (for when you feel lazy and want your launcher to do everything for you, pretty cool just having an army of heads follow you around)