The first thing I notice when I see your team is that a flying type would demolish half of your team, as well as the fact that you lack a special attacker. Also, your EV spreads are very bad, becasue if you spread your EVs to thin, they won't make much of a difference. This team isn't bad but it could be a lot better.
Gigalith - you have a good set here, except for iron defense, if your going to use explosion it is pointless to have iron defense, because it would make no sense to boost up your stats and then blow them to peices. Also, a rock gem is kind of pointless, as it goes away after the first use. I would suggest somehting like this:
Gigalith - Lead - Leftovers
EVs: 252 attack, 252 defense, 4 HP
Stone Edge (STAB)
Stealth Rock (Entry hazard)
Explosion (He's not going down without a fight, andexplosion lets him go out with a bang)
This gigalith is a good stealth rock user, with his amazing defense and attack, he can tank a physical move, set up some rocks, and then blow the opposition into smithereens.
Conkeldurr - your set isn't bad but every thing else is garbage. You have 200 special attack EVs, and you dont even have any special attacks, which is pointless. Also, the fist plate is a bad item here, as he only has one fighting type move. I would try this set:
Conkledurr - Physical sweeper - Toxic orb
EVs: 252 HP, 252 Attack, 4 speed
Drain Punch (STAB ,HP regain, guts boost)
Stone edge (Coverage, guts boost)
Mach Punch (STAB, priority)
Conkeldurr is a powerhouse with (and without) guts boost, which is like giving him a choice band without locking him into a move. One more nice thing is that drain punch will help regain some HP (and you better believe hes got a lot of it).
Machamp - again with the EVs, what are you thinking? With conkeldurr on your team machamp is just a liability, one more flying weakness. the presence of 2 mono fighting types just means trouble for you, so I would replace machamp with this:
Starmie - Special Sweeper - Life orb
EVs: 252 special attack, 252 speed, 4 HP
Ability: Natural Cure
Scald/Surf (STAB, Burn VS. power)
Recover/Rapid Spin (HP regain)
Starmie serves as a special attacker, and a great one too. most of the strategy is just to pound the target with attacks.
Leavanny - Might I suggest that you get someone to tutor you in EVs, because your EV spreads are bad, but you will get better over time. Now, the scarf is fine on him, but not with grasswhistle, as you will be locked into that move, which would be kinda pointless, so I would try this set:
Leavanny - Revenge killer - Choice Scarf
EVs: 252 attack, 252 speed, 4 HP
Leaf Blade (STAB)
Shadow claw (coverage)
Aerial ace (coverage)
Gyarados - Not going to mention the EV spread, dragon gem is kinda trash, as it will be gone on the first use of outrage. try this set:
Gyarados - Special Sweeper - Lum Berry
EVs: 252 special defense, 252 HP
Thunder Wave (Cripples the foe)
Dragon Tail (Allows him to parashuffle)
Substitute (Lets him work more smoothly)
The idea here is that you paralyze the opposition with thunder wave, and then use dragon tail to switch the cripples enemy for a new one, and then you just repaet the cycle.
Darmanitan - The EVs are yet again messed up, but that's not too big of adeal as lonfg as you change them. I'm not sure why you have him holding a flame orb, and 2 (even 1) normal move(s) is really not neccessary. I would try this:
Darmanitan - Physical Sweeper - Choice Scarf
EVs: 252 attack, 252 speed
Ability: Sheer force
Flare Blitz (STAB, sheer force boost)
Stone Edge (Sheer force boost, coverage)
U-turn (Lets him get a clean getaway)
Superpower (Sheer force boost, coverage)
This set lets darmanitan take advantage of his ability, and he hits pretty hard with it too.