"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, such as Yuri, Dennis McField, Jean-Michel Roget and the Obelisk Force members, in Yu-Gi-Oh! ARC-V. All monsters of this archetype are EARTH, except for the DARK "Chaos Ancient Gear Giant".
"Ancient Gear Gadjiltron" monsters share a relation with the "Gadget" archetype, gaining additional effects when "Red", "Green" and/or "Yellow Gadget" are Tributed to Tribute Summon them. This relationship is further extended with their own Gadget monster in "Ancient Gear Gadget", as well as the monster, "Ancient Gear Reactor Dragon".
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.
The signature feature of the "Ancient Gear" monsters is to overwhelm the opponent while preventing them from activating Spell or Trap Cards when they attack, largely because Trap Cards and Quick-Play Spell Cards which activate during the Battle and Damage Steps tend to be some of the most common cards used by players, and the "Ancient Gear" archetype focuses upon preventing the opponent from using them. The concept of having limited Spell and Trap immunity is similar to the Duelist Kingdom arc of the Yu-Gi-Oh! anime, in which Machine monsters were immune to magical attacks. This allows "Ancient Gear" monsters to attack without fear of being stopped by any Spell/Trap Cards.
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. Due to the prevalence of monster effects that can be activated during the Battle Phase, the "Ancient Gear" monsters introduced in Structure Deck R: Machine Dragon Re-Volt can prevent their activation too.
"Ancient Gear Gadjiltron Chimera" and "Ancient Gear Gadjiltron Dragon" can be used in combination with the "Gadget" monsters to unleash their effects. Since all of the "Ancient Gear" monsters are Machine, the Spell Cards "Limiter Removal" and "Solidarity" can be necessary.
Both "Ancient Gear Reactor Dragon" and "Ancient Gear Hydra" have reliance on Tribute Summons, gaining effects upon Tributing either "Ancient Gear" or "Gadget" monsters although can still function as competent threats even if just Special Summoned, as the former can still destroy Spell or Trap Cards upon attacking while the latter can provide total protection upon attacking to any of the "Ancient Gear" monsters the player controls. "Catapult" is best used to destroy "Geartown" or other cards like "Ancient Gear Fortress" in order to fuel the swarm. Since "Catapult" ignores the Summoning conditions, it can also Special Summon "Beast" or "Golem". Also, the effect of "Reactor Dragon" can be used on one's own "Geartown" or "Fortress" to obtain a free monster during the Battle Phase where the opponent is fundamentally prevented from playing.
Due to their focus on Tribute Summoning and destruction a few techs can be used to great effect. For example, "Gold Gadget" and "Silver Gadget" can be used due to their ability to float and eventually Summon "Ancient Gear 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". Doing so can reap twice the benefits as the player can have both a Boss monster with its full effects and at the same time they can use the "Metalfoes" cards to further expand their strategy. However, one should be careful with this strategy, as if the player uses the effect of "Wyvern", the "Metalfoes" Pendulum Cards can no longer use their effects for the remainder of the turn (they can if they do so before using "Wyvern"), so while the mix can work well, caution is advised.
With a few exceptions, "Ancient Gear" monsters are not immune to Spell/Trap Cards, so attacking with a non-"Ancient Gear" monster will make them vulnerable to cards like "Mirror Force".
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".