"Ancient Gear", known as "Antique Gear" (アンティーク・ギア Antīku Gia) in the OCG, is an archetype of Machine monsters used by Vellian Crowler in Yu-Gi-Oh! GX, Rudolph Heitmann in Yu-Gi-Oh! 5D's and Duel Academy students' personnel, such as Yuri, Dennis McField, Jean-Michel Roget and the Obelisk Force members, in Yu-Gi-Oh! ARC-V. All "Ancient Gear" monsters, except "Chaos Ancient Gear Giant", are EARTH monsters.
The humanoid "Ancient Gears" bear a striking similarity to Ancient Greek soldiers, and so their creation may be linked to the Antikythera mechanism.
True to their name, all "Ancient Gear" monsters resemble armored robots of varying designs with their bodies featuring unions of cogwheels and gears, joining several parts of their bodies together. The backgrounds of most of the Main Deck monsters features gears.
Their name "Ancient Gear" is likely a reference to the old age of the mechanism that governs their functioning, which normally would require an external force to move them but interestingly enough they seem to be self-sufficient. This implies that contrary to their apparent antiquity, that they may be far more advanced than they look.
When they are used as Fusion Material by Duel Academy members in Yu-Gi-Oh! ARC-V, their Summon Chant mentions that they carry "ancient spirits" or other similar terms. This seems to imply that the "Ancient Gear" monsters may be old machines possessed by spirits, which could explain their functioning despite their old workings. This could also be the reason behind their glowing eyes.
"Ancient Gear" Decks focus on either Tribute Summons or Fusion Summons. The signature feature of the "Ancient Gear" monsters is to prevent the activation of Spell/Trap cards during their attack. This allows "Ancient Gear" monsters to attack without fear of being stopped by any Spell/Trap Cards. Due to the prevalence of monster effects that can be activated during the Battle, newer monster also prevent their acivation. (They aren't immune to Spell/Trap Cards, however.)
A number of cards allow "Ancient Gear" monsters to be Summoned from the Deck, such as "Spell Gear", "Ancient Gear Catapult", "Geartown" and "Machine Duplication". Cards such as "Geartown" and "Ancient Gear Castle" allows ease of Tribute Summoning "Ancient Gear" monsters. "Catapult" is best used to destroy "Geartown" or "Ancient Gear Fortress" in order to fuel the swarm. Since "Catapult" ignores the Summoning conditions, it can also Special Summon "Ancient Gear Beast" or "Ancient Gear Golem (- Ultimate Pound)".
For their supports, "Geartown" and "Ancient Gear Castle" ease Tribute Summon, and the latter boosts the ATK. "Ancient Gear Fortress" and "Ancient Gear Hydra" provide a shield to "Ancient Gear" cards, and "Ancient Gear Reborn" can easily revive those monsters from the GY as well as strengthen them slightly. "Ancient Gear Wyvern" and "Ancient Gear Workshop" can allow for hand recovery.
For Fusion Summons, "Ancient Gear Hunting Hound" is simply "Polymerization". The series starts off with "Ancient Gear Howitzer", which can inflict damage and summon more "Ancient Gear" monsters while ignoring the Summoning conditions when destroyed by battle. "Megaton Ancient Gear Golem" can allow for multiple attacks (should the right monsters be used as Fusion Material), and can then evolve into "Ultimate Ancient Gear Golem" for more damage, and finally, the only non-EARTH monster, "Chaos Ancient Gear Giant", for total eradication. The deck further encourages the use of "Ancient Gear Golem" with their signature Fusion card "Ancient Gear Fusion", which can almost instantly bring out any of the "Ancient Gear" Fusion Monsters if "Golem" is on the field.
This deck has synergy with "Gadget" archetype, as "Ancient Gear Gadjiltron Chimera" and "Ancient Gear Gadjiltron Dragon" can be used in combination with the "Gadget" monsters to unleash their effects. Both "Ancient Gear Reactor Dragon" and "Ancient Gear Hydra" also gain effects upon Tributing either "Ancient Gear" or "Gadget" monsters while still functional even just Special Summoned. The relationship is further strenghthened by "Ancient Gear Gadget", a monster that is of both archetype at the time, with the monster itself can act in place of any "Gadget" monsters in the card pool.
"Gold Gadget" and "Silver Gadget" can be used due to their ability to float and eventually Summon "Gadget", which upon Tributing can provide "Reactor Dragon" and "Hydra" with their full effects. "Gold" and "Silver Gadget" can also be used in conjunction with "Ancient Gear Wyvern" in order to search a card and at the same time Xyz Summon "Gear Gigant X" for further searching.
Other cards can work well with the Deck are Pendulum Cards, specially the "Metalfoes" archetype. This is mostly because the Pendulum monsters can be placed on a Pendulum Zone, making them valid targets for "Catapult", also "Metalfoes" themselves can destroy "Geartown" or "Fortress" if needed, and can also destroy "Gold" and "Silver Gadget" with minimal drawbacks while at the same time fueling their main strategy. Moreover since they can enable Pendulum Summons they can greatly facilitate the Tribute Summon of "Reactor Dragon" or "Hydra" and if "Gold and Silver Gadget" are Special Summoned this way, they can be destroyed so the player can search both "Metalfoes" cards and "Ancient Gear Gadget". However, the use of "Wyvern" prevents the use of "Metalfoes" cards.
The New Master Rules, however, hardly affect the deck itself (excluding non "Ancient Gear" teched in). As the player could focus on Tribute Summon "Golem" or "Reactor Dragon", which is as strong as Extra Deck Monster. Also, the deck would only need one of their Fusion Monster at a time.
Cards like "Acid Rain", "Magnetic Mosquito", and "System Down" have the potential to shut down an "Ancient Gear" Deck completely. Although these cards can be hard to counter, "Imperial Iron Wall", "Starlight Road", "The Huge Revolution is Over" and other Spell/Trap negation cards can prevent them activating.
Finally, being Machine Monsters, they can be used as Fusion Materials for an opponent's "Chimeratech Fortress Dragon".