The theory of Gadgets.
#1: They're floaters. One of them searches another. Once you summon it, it already paid for itself by adding another Gadget Monster. Think of Gadget decks as if your foe is running 9x Sangans. Yeah, that's scary, and that's the concept.
#2: So basically, it's card advantage power over Monster stats. It's all about swarming with weenie monsters for direct attack. I mean, a monster with a bunch of 1200~1400ATK is a poor monster field presence, until you got a mass army that you can't get rid of that is. So in a Gadget deck, you draw a card, and then you summon a Gadget to add another gadget to your deck. So in that sense, you draw 2 cards per turn for 9 turns. (i.e. until you run out of Gadgets) Add in Pot of Avarice, and these "2 card draw per turn" can be extended by refueling your deck with gadgets that's already in the graveyard. That's ALOT of card advantage. If a Gadget decks goes well, their field and hand will be filled with cards, too many cards for the player to handle and can pretty much waste them extra for game.
#3: But like I said, the weakness of gadgets are poor stats. To fill that up, you run alot of 1 for 1 card trade. This includes like Smashing Ground/Fissure/Saku/Widespread/BTH/Torrential/Mirror/Ring/etc. etc. etc. (Most Gadget decks runs likt 9~11 traps. And they all max 3x Smashing Ground and then add in even Fissure to boot. That's how much of a 1 for 1 trade dedication Gadget Decks have.)
#4: Finally, if you want to run 9x of all Gadgets, they usually run a 45 fat deck. Mainly, cause you don't want to "draw" your Gadget once you have one in your hand. You always want a card that's NOT a Gadget being drawn, and then just summon a Gadget each turn and search a Gadget into your hand that way. Cause if not, you'll draw like 2 Gadget per turn and your deck dies cause you have nothing but Gadgets all stuck in your hand. That's not card advantage, that's dead hand. To decrease the chance, they increase the deck size. In fact, Gadget decks are the only decks that actually works well with a fat 45 deck.
#5: Lastly, everybody have their own twist of their own Gadget Decks. Sometimes, regular 1 for 1 trade isn't enough, and Gadgets can fail you. So Gadgets sometimes have alternative win conditions. For some, they run Pot of Avarice for "refuel-retry". Some uses Ultimate Offering for the OTK. Other can use Overload Fusion instead of PoA for Chimera OTK. (Gadgets fills the graveyard FAST) Some others even run Future Fusion to make sure that Cyber Dragon is in the graveyard for Overload use. Some run Enemy Controller/Rush Recklessly while others run Fissure/Hammer Shot instead. Some decks don't run Sangan while some still do. Some run Spirit Reaper while others don't bother.
Anyway, this is my example take of a gadget deck.
Tribute - 3
Cyber Dragon x3
Non-Tribute - 15
Breaker
Chiron the Mage
Exiled Force
Green Gadget x3
MoF
Red Gadget x3
Snipe Hunter x2
Yellow Gadget x3
Spells - 17
Confiscation
Enemy Controller x2
Fissure
Future Fusion
Graceful Charity
Heavy Storm
Limiter Removal
MST
NoC
Overload Fusion x2
Rush Recklessly x2
Smashing Ground x3
Traps - 10
BTH x2
Mirror
Ring
Saku x3
Torrential
Ultimate Offering
Widespread Ruin
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2 of Top 8 Shounen Jump that used a Gadget deck. (3rd and 4th place)
Chris Evans
Monsters: 20
2 Snipe Hunter
2 Cyber Phoenix
1 Magician of Faith
3 Cyber Dragon
1 Exiled Force
3 Red Gadget
3 Green Gadget
3 Yellow Gadget
1 Breaker the Magical Warrior
1 Chiron the Mage
Spells: 16
3 Smashing Ground
2 Enemy Controller
1 Limiter Removal
1 Nobleman of Crossout
1 Last Will
2 Rush Recklessly
1 Confiscation
1 Fissure
1 Mystical Space Typhoon
1 Overload Fusion
1 Heavy Storm
1 Graceful Charity
Traps: 9
1 Torrential Tribute
3 Sakuretsu Armor
1 Ring of Destruction
2 Bottomless Trap Hole
1 Ultimate Offering
1 Mirror Force
Side: 15
2 Trap Dustshoot
2 Royal Oppression
1 Lightning Vortex
1 Overload Fusion
1 Future Fusion
2 System Down
1 Brain Control
1 Chiron the Mage
2 Mobius the Frost Monarch
2 Des Wombat
John Brewer
Monsters: 19
3 Cyber Dragon
3 Red Gadget
3 Yellow Gadget
3 Green Gadget
2 Banisher of the Radiance
2 Snipe Hunter
1 Breaker the Magical Warrior
1 Kycoo the Ghost Destroyer
1 Exiled Force
Spells: 17
1 Graceful Charity
1 Nobleman of Crossout
3 Smashing Ground
1 Confiscation
1 Pot of Avarice
3 Fissure
1 Heavy Storm
2 Hammer Shot
1 Mystical Space Typhoon
1 Limiter Removal
1 Premature Burial
1 Enemy Controller
Traps: 9
1 Mirror Force
1 Torrential Tribute
3 Sakuretsu Armor
1 Ultimate Offering
2 Widespread Ruin
1 Ring of Destruction
Side Deck:
2 Des Wombat
1 Chiron the Mage
1 D.D. Designator
1 Wave-Motion Cannon
1 Exchange
1 Lightning Vortex
2 Royal Decree
2 Trap Dustshoot
1 Chain Destruction
2 Bottomless Trap Hole
