"Ancient Gear Soldier" driving an "Ancient Gear Tank"
"Ancient Gear", known as "Antique Gear" (アンティーク・ギア Antīku Gia) in the OCG, is an archetype of Machine-Type monsters that are used by Vellian Crowler in Yu-Gi-Oh! GX and Rudolph Heitmann in Yu-Gi-Oh! 5D's. They overwhelm the opponent while preventing them from activating Spell or Trap Cards when they attack. They were released in The Lost Millennium, Shadow of Infinity, and Structure Deck 10: Machine Re-Volt, with further support in Light of Destruction and The Duelist Genesis.
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 your opponent from using them. The concept of having limited Spell and Trap immunity is similar to the Duelist Kingdom arc of the second series anime, where Machine-Type monsters were immune to magical attacks.
The humanoid "Ancient Gears" bear a striking similarity to Ancient Greek soldiers, and so their creation may be linked to the Antikythera mechanism. The four original "Ancient Gear" monsters: "Beast", "Engineer", "Golem", and "Soldier" all have ATK equal to their DEF. "Cannon" also follows this rule.
A number of cards allow them to be Summoned from the Deck exist, such as "Spell Gear" and "Machine Duplication". Most "Ancient Gear" monsters require Tributes to Summon. "Ancient Gear Workshop" are also recommended if you use "Ancient Gear Golem" and "Ancient Gear Beast", as they cannot be Special Summoned.
"Ancient Gear Drill" can be used to search out some Quick-Play Spell Cards like "My Body as a Shield", "Shrink", and "Limiter Removal". These cards work well with "Ancient Gear Drill" as they can be activated outside your own turn. Cards like "Ancient Gear Castle" and "Geartown" to make Normal Summoning the Level 5 or higher "Ancient Gear" monsters easier.
"Ancient Gear Gadjiltron Chimera" and "Ancient Gear Gadjiltron Dragon" can be used in combination with the "Gadget" monsters to unlock power effects. Since all of the "Ancient Gear" monsters are Machine-Type, the Spell Cards "Limiter Removal" and "Solidarity" can be necessary.
"Ancient Gear" monsters are not immune to Trap Cards (aside from targeting "Ancient Gear Engineer"). If your opponent has a Set "Mirror Force", for example, attacking with a non-"Ancient Gear" monster will allow your opponent to destroy all of your Attack Position monsters. Another vulnerability to look out for are Monster destruction/banishing effects.
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. Again, "Ancient Gear Beast" can negate Flip Effect Monsters such as "Magnetic Mosquito".
Finally, being Machine Monsters, they are especially vulnerable to "Chimeratech Fortress Dragon", which can allow your opponent to clear your field and gain a 2000+ ATK beatstick. For this reason, you should never leave a "Cyber Dragon" on your side of the field whilst you control another machine. It may also be wise to add "Battle Faders" to your deck to prevent your opponent from attacking directly, as well as giving you tribute fodder for the higher level Ancient Gears. Combine this with cards that change battle positions such as "Enemy Controller" or reduces ATK such as "Shrink" and you could easily turn the tables to your favor.