Competitive Team Help

So here's another team I came up with and I want to know how to make it better in any way. I do want to keep away from having a team of really popular pokemon so please take that into consideration. I would also like to keep my Gigalith and the explanation is at the bottom after all the information.

Durant
Item: Scope Lens
Nature: Adamant
Ability: Swarm
EV: 252-Speed, 252-Attack, 4-Special Defense
Stone Edge
Iron Head
X-Scissor
Crunch

Mienshao
Item: Black Belt
Nature: Adamant
Ability: Regenerator
EV: 252-Speed, 252-Attack, 4-HP
Drain Punch
U-turn
Stone Edge
Poison Jab

Galvantula
Item:Zap Plate
Nature: Modest
Ability: Compoundeyes
EV: 252-Speed, 252-Special Attack, 4-HP
Electro Ball
Energy Ball
Bug Buzz
Thunder Wave

Snorlax
Item:Chesto Berry
Nature: Impish
Ability: Immunity
EV: 252-Attack, 252-HP, 4-Speed
Return
Rest
Belly Drum
Zen Headbutt

Cloyster
Item: Focus Sash
Nature: Naughty
Ability: Skill Link
EV: 252-Attack, 252-Speed, 4-HP
Ice Shard
Rock Blast
Shell Smash
Icicle Spear

Gigalith
Item: Leftovers
Nature: Adamant
Ability: Sturdy
EV: No EV training (Explanation at bottom)
Return
Stone Edge
Earthquake
Rock Polish

So about that Gigalith... I've tried EV training different Gigalith before and nothing I tried came out as good as my original Gigalith. I don't know what it is, but it does very well and people have come up to me at tournaments and told me that they're scared of my Gigalith. So that's why I would like to keep it. Plus it's shiny!

Thank you for reading and any help will help!

Keep Gigalith and drop Durant and Galvantula. Instead of Durant get a Dragon, ghost, psychic type pokemon. For galvantula get a magnemite and evolve it. Keep snorlax and Meinshao, Cloyster

Well first of all, I juuuust want to ask, what's you're general direction of your team? Is there a specific plan you had in mind? Or a specific gamestyle? Well, it kind of seems that you'd want to go all out offense, even though there are specific pokemon that can perform the roles that your pokemon holds better, but, if you say you don't want to use popular pokemon, than it's up to you.

Buuut just a few suggestions:

I would use Life Orb > Scope Lens for Durant. Life Orb is overall the better choice of item despite this decreasing Durant's survivability. While Hustle increases your Durant's attack stat by 50%, Life Orb will also increase your attack by 1.3. So, that will mean that assuming that your Durant has max attack (348), it'll increase by 50%, and the attack will times by 1.3, which will come about to 679 Attack, giving you more opportunities to 2HKO - OHKO certain pokemon without relying on a crit. Also, try Hone Claws > Crunch , since Hustle increases Durant's Attack but lowers its accuracy, which makes Hone Claws a great move for it; Hone Claws not only increases its accuracy, but also its Attack, which are both key elements for a sweep.

If you want to use Mienshao, try using Life Orb > Black Belt, because not only it will give Mienshao more power, Mienshao can recover the recoil damage from Life Orb with Regenerator anyway. I also recommend Choice Scarf if you want to use an Adamant nature, just to patch up it's speed. I prefer using Hi Jump Kick > Drain Punch for a stronger fighting move, but that's up to you; both moves can work fine. I'd also advice using Aerial Ace > Poison Jab, perhaps Fake Out > Poison Jab, or if you can, Hidden Power [Ice] > Poison Jab. Poison moves aren't that versatile in competitive play (besides Toxic, Toxic Spikes, or maybe Sludge Bomb), and so, if you stick to moves like Aerial Ace, you'll gain more coverage (against Fighting, Bug, Grass). Hidden Power Ice can hit things like Gliscor or Salamence as well.

For your Galvantula, I recommend something like Life Orb > Zap Plate or Choice Specs > Zap Plate. Again, they provide more power for your other moves. I also recommend Thunder > Electro Ball. With a Modest nature, you'll won't be that fast contending with certain pokemon, even if it's base speed is 108. So, you'll miss out on a lot of OHKOs. Now with Thunder, while normally it's inaccurate, Compoundeyes will boost it straight up to 91% accuracy, in which Thunder is also a far better move on picking up 2HKOs or even OHKOS. I also recommend Giga Drain > Energy Ball, because healing yourself is far more efficient, and maybe something like Volt Switch > Thunder Wave, because you don't need something like Thunder Wave if you're using Thunder, and plus, Volt Switch + U-turn from your Mienshao is a really great combination for scouting, and for keeping momentum.

Now, Snorlax - it's far too slow for a Belly Drum set, unless you're using Trick Room, which you don't have. So, why don't you try a Choice Band set? Normally, a Snorlax with Choice Band has these moves:

~ Return / Body Slam
~ Earthquake
~ Pursuit
~ Fire Punch / Crunch

You don't need Zen Headbutt, because you'll ultimately want to maximize your power, and give you better coverage options. With your EVs, it can definitely make it possible. However, this set will prefer you to use Adamant > Impish. Then again, if you want to stick with Rest, with an Impish nature, try a Rest Talk set:

~ Body Slam / Return
~ Rest
~ Sleep Talk
~ Whirlwind

That way, you'll have better surviviability, and Whirlwind can be handy of Phasing out pokemon that likes to set-up. But, it's just a matter of your preference.

And well, that's all for my suggestions. Again, there are good pokemon out there that can perform the roles better than the pokemon you've listed, such as Rotom-W > Galvantula, and Jirachi > Durant. But again, it's all up to you on what you want to do. I just recommend you check out some other pokemon too, and see if you like any pokemon that can fit in your team better. I recommend checking out smogon's competitive pokemon list as they explain a wide variety of pokemon, and their roles, and their movesets.

Hope this helps, and good luck to your future battles. :3

To add to what Doronjo said, and to condense his set suggestions into simpler terms, Jolly is the preferred nature on Durant, as Hustle makes it stronger than it already is. Superpower is also worth considering to hit Steels, as Durant had a hard time getting past them outside of the unreliable Dig in B1/W1:
-Hone Claws
-X-Scissor
-Iron Head/Superpower
-Rock Slide/Superpower
Nature: Jolly
EVs: 4 HP/252 Atk/252 Spe
Item: Life Orb/Lum Berry
Ability: Hustle

Always use a +Speed nature on 'mons with high Speed, as those Pokémon appreciate the slight Speed boost to outspeed as much as they can; Mienshao is no exception. If this is an in-game team, it's fine if you don't have HP Ice, as it could take forever to get the right IVs for that type:
-Hi Jump Kick
-U-turn
-Hidden Power (Ice)/Stone Edge
-Fake Out/Grass Knot
Nature: Naive/Jolly
EVs: 252 Atk/4 SAtk/252 Spe
Item: Life Orb
Ability: Regenerator

Snorlax has a hard time setting up Curse in OU because of the fast pace there; the OU environment is also littered with Fighting Pokémon like Terrakion and Conkeldurr. It's better on the offensive in OU:
-Return/Body Slam
-Pursuit
-Fire Punch
-Superpower/Wild Charge
Nature: Adamant
EVs: 72 HP/252 Atk/184 SDef
Item: Choice Band
Ability: Thick Fat

Cloyster is better off with a Naive nature; as I said before, the Speed boost from Naive is important and Naughty fails to outspeed some Choice Scarfed threats even after +2 Speed.

As much as you want to keep Gigalith, let me say this: Gigalith sucks in OU. Seriously. Because of its common weaknesses, very shallow movepool and the fact that it's seriously outclassed by virtually every Rock Pokémon around it (Tyranitar has a better movepool, Golem has Sucker Punch for hitting faster threats, Aggron has recoil-free Head Smash to muscle past most opponents, Rhyperior has Solid Rock to cushion supereffective hits, Terrakion has power and Speed etc.). In fact, Gigalith's Speed is also so terrible that not even Rock Polish can save it. Your best bet here is Terrakion:
-Stealth Rock
-Stone Edge
-Close Combat
-Swords Dance/Taunt
Nature: Jolly
EVs: 252 Atk/4 SDef/252 Spe
Item: Focus Sash

or
-Swords Dance
-Rock Polish
-Close Combat
-Stone Edge
Nature: Jolly/Adamant
EVs: 4 HP/252 Atk/252 Spe
Item: Life Orb/Air Balloon

or
-Close Combat
-Stone Edge
-X-Scissor
-Sacred Sword/Rock Slide/Quick Attack
Nature: Jolly
EVs: 4 HP/252 Atk/252 Spe
Item: Choice Scarf/Choice Band

or
-Substitute
-Swords Dance
-Close Combat
-Stone Edge
Nature: Jolly
EVs: 252 Atk/4 Def/252 Spe
Item: Life Orb/Salac Berry/Rock Gem

Edit: even after the changes me and Doronjo made, you have a bad weakness to Technician Breloom. Volcarona is a big help here, as it resists its STABs (though it must watch out for Stone Edge or Spore):
-Quiver Dance
-Fire Blast/Fiery Dance
-Bug Buzz
-Hidden Power (Ground/Rock)/Giga Drain
Nature: Timid
EVs: 252 SAtk/4 SDef/252 Spe
Item: Life Orb

or
-Quiver Dance
-Bug Buzz
-Fiery Dance/Fire Blast
-Roost
Nature: Timid
EVs: 240 HP/252 Def/16 Spe
Item: Leftovers

or
-Quiver Dance
-Fire Blast/Fiery Dance
-Bug Buzz
-Rest
Nature: Modest
EVs: 160 HP/236 SAtk/112 Spe
Item: Chesto Berry

Thank you guys for your help and I will take these suggestions into serious consideration.