"Green Gadget", "Red Gadget" and "Yellow Gadget".
There are two main groups of "Gadget" cards. The first is the colored "Gadgets" ("Green Gadget", "Red Gadget" and "Yellow Gadget"), who can add another specific colored "Gadget" monster from the Deck to the hand upon being Summoned. The second group ("Gadget Arms", "Gadget Hauler", "Gadget Driver") are "Morphtronic" support. "Gadget Soldier", who is not part of either group, is also part of this archetype. The colored "Gadgets" and the only "Gadget" support card were released in Structure Deck: Machine Re-Volt.
Yugi used the colored "Gadgets" in the The Ceremonial Battle at the end of the Yu-Gi-Oh! anime and manga, Seto Kaiba and Kozaky used "Gadget Soldier", and Leo utilized the "Morphtronic" support "Gadgets" in Yu-Gi-Oh! 5D's.
Colored "Gadget" monsters
The background art of each "Gadget" references both the "Gadget" they search out and the "Gadget" that searches them; the gears in the background refer to the former, and the energy lines around the "Gadget" refer to the latter. The eye color references the "Gadget" they search out as well.
Since their release, "Gadget" monsters have gained synergy with the "Ancient Gear" archetype due to "Ancient Gear Gadjiltron Chimera" and "Ancient Gear Gadjiltron Dragon," which require Tribute Summoning by Tributing Gadgets of particular colors to gain certain effects. As "Gadgets" are all EARTH-Machines, like "Ancient Gears", as well as the fact that "Ancient Gear" monsters need ready access to Tribute Fodder, the two archetypes merge well. They also work well with the "Machina" archetype for what is known as the Machina Gadget Deck.
Colored "Gadget" monsters
A "Gadget" Deck is a Deck Type that is designed to use the effects of the "Gadget" monsters ("Red Gadget", "Yellow Gadget" and "Green Gadget") to gain Card Advantage. You can Summon one "Gadget"and add another to your hand, to get ahead in card advantage and use 1 For 1 Cards to remove as many opposing cards as possible while staying ahead with extra cards you added to your hand with each "Gadget".
The third way is to clear the field of monsters with cards like "Torrential Tribute" or "Lightning Vortex", use "Ultimate Offering" to Summon five "Gadgets" to the field, then activate "Gaia Power" to tack on an additional 2500 points of damage, or "United We Stand" for an additional 4000 damage and possibly the win.
The fifth is to Summon "Beast King Barbaros", tributing 3 "Gadgets" Summoned with "Ultimate Offering" and destroying all cards on the opponents side of the field, then Summoning 4 more and attacking directly for a win.
The seventh way is to use Ultimate Offering to Summon a field of Rank 4 Xyz Monsters since each "Gadget" searches out for another (you could Summon up to 4 in a turn, in theory). The eighth way is to once again use Ultimate Offering to swarm with "Gadgets" and to Tribute them for Monarchs and clear the field. This isn't likely as you would need 4 "Monarchs" in your hand.
The eighth way is quite risky, due to the fact that you are removing from play many of your monsters, and should be used as a game ender. Needless to say, Ultimate Offering being in 7 out of 8 of the above OTKs, it is a staple in Gadget Decks, being able to swarm and quickly win the game. However only one copy of Ultimate Offering can be run in a deck which can make it difficult to draw.
With lots of support from the Machina Mayhem Structure Deck, Machina "Gadgets" focus on getting out "Machina Fortress" by dumping "Machina Force" or any monsters whose Level equal Level 8 or more by taking advantages of your "Gadgets". Machina "Gadgets" can easily search out "Machina Fortress" by using the effect of "Machina Gearframe". "Machina Peacekeeper" also helps since it can search out "Machina Gearframe" and then "Machina Fortress", however "Machina Peacekeeper" is not preferred as it slows down the Deck and could possibly become a dead draw at some point. Cards like "Pot of Avarice", "Daigusto Emeral", and "Scrap Recycler" are useful to this kind of Deck since you can re-use your "Gadgets" so that you may discard them to Summon "Machina Fortress".
Oppression Gadget Decks are Gadget Decks that use cards such as "Royal Oppression" to negate the Special Summons of your opponent. One-for-one cards like "Bottomless Trap Hole", "Dimensional Prison", "Smashing Ground" and so on are also used in this Deck to clear the field of your opponent's cards. "Macro Cosmos" should be a great combo to this Deck, since your opponent would have no access to the Graveyard, thus preventing strategies like those active in the Graveyard from working. This Deck can only be used in Traditional Format duels as "Royal Oppression" is banned in Advanced Format duels.
Use "Zombie World" to turn the "Gadgets" into Zombie-Type monsters. This will allow the "Gadgets" to be able to use the standard Zombie support if combined with "Card of Safe Return" (traditional format only), this will create very large amounts of Deck Thinning. The "Gadgets" can then be used with "Plaguespreader Zombie" in order to Summon powerful Synchro Monsters
Using "Cyber Dragon" with the "Gadgets" and with one-for-one removal cards to clear the opponent's field in order to Summon an overpowered "Chimeratech Overdragon" or "Chimeratech Fortress Dragon" and possibly also "Cyber Twin Dragon" and "Cyber End Dragon". "Cyber Dragon Zwei" and "Proto-Cyber Dragon" can be used as substitutes for "Cyber Dragon".
Using the "Gadgets" in a Psychic Deck is a great way to develop early field control. Using the "Gadgets" combine with the Spell card "Emergency Teleport" on the first turn can give you a any where from a Level 5 to 7 Synchro Monsters. By using "Emergency Teleport" to Summon "Krebons" or "Psychic Jumper" you can easily get "Goyo Guardian" or "Brionac, Dragon of the Ice Barrier" (both are in traditional format only), and since this is combine with the search effects of the "Gadgets", it will add an extra card to your hand to discard for the effect of "Brionac, Dragon of the Ice Barrier". ability.
This Deck uses "Gadgets" and "Ultimate Offering" (now Forbidden) to Summon various monsters that use three Tributes and overwhelm the opponent. Various cards that may be used include: the legal Egyptian Gods, the Wicked Gods or even "Beast King Barbaros", "Gilford the Lightning", and "Destiny Hero - Plasma".
A Ritual Gadget Deck uses the "Gadgets" as quick Ritual material for "Demise, King of Armageddon" and "Ruin, Queen of Oblivion". By playing one "Gadget", you bring another to your hand, giving you 2 Level 4 monsters to Tribute for "End of the World". This Deck is easy to build and can quickly bring Demise to the field, often on the first turn.
This Deck is based on the power of "Ancient Gear" with the relevant support of the "Gadgets". Both have synergy in this Deck, and they may use the special EARTH support "Spiritual Earth Art - Kurogane". If you destroy "Geartown" you'll be able to Special Summon the Ancient Gear powerhouse "Ancient Gear Gadjiltron Dragon".
Using cards like "Double Summon", "Ultimate Offering" and "Tin Goldfish", bring out two or more "Gadgets" to instantly Summon a Rank 4 Xyz Monster. It is easy to Summon up to 4 Xyz Monsters in a turn using "Ultimate Offering". Alongside packing your Extra Deck, use "Doomcaliber Knight" and "Thunder King Rai-Oh", 1900 ATK beatsticks with Anti-Meta effects and can be used for a Xyz Summon. These add an element of control to the Deck. "Gogogo" monsters also work well in this "Deck". Also, cards like Reborn Tengu and Summoner Monk will special summon an extra monster from the deck for easy Xyz Summonning, deck thinning and field advantage.
It is worth combining the "Gadget", "Crashbug" and "Exodia" cards as the "Crashbugs" and "Gadgets" decrease your Deck size so you stand a better chance of drawing the "Exodia" cards. You can treat this Deck as a normal "Exodia" Deck so you can still use "One Day of Peace" for example.
While this Deck can be strong, cards like "System Down" and "Acid Rain" will destroy all three "Gadgets" and all "VWXYZ" monsters. "Malfunction", "Mystical Space Typhoon", "Royal Decree", and "Trap Reactor・Y FI" (which also inflicts burn damage) would prevent "Stronghold the Moving Fortress" from appearing in the field. Sliding cards like "Imperial Order" and "Imperial Custom" (try to get this card out instead) can counter these cards.