Rank 4 engine:
Geargiarmor: A good defense card and a great searcher who albeit being a bit slow works fantastically if not destroyed. Because it can be flipped back down, it is a reusable searcher who if used a second time already is netting huge advantage.
Geargiaccel: A card which more or less makes the deck what it is. Geargiaccel can be summoned by merely having another Geargia monster on the field and hence, it is incredibly good for a Rank 4 engine such as that which Geargias have. It works well with Geargiarmor because if you flip Geargiarmor and get Geragiaccel, that is a fast and free rank 4 XYZ monster.
Geargiarsenal: Another incredibly good search monster which can be tributed in order to summon a Geargia Monster from the deck and hence get the Geargiarmor necessary to set up the Geargiarmor/ Geargiaccel play.
Gear Gigant X: the boss monster of the archetype, Gear gigant X allows for enormous pluses for when you detach a XYZ material from it, once per turn you can get any level 4 machine time monster to your hand... that is like the gadgets on crack. This card alone is a huge addition to the machine type strategy.
Rank 3 engine:
Geargiano: not to great a card but it works ok with cards which tribute as a cost and then activate like geargiarsenal. This is really only there to supplement Geargiano MK-II.
Geargiano MK-II: This card allows for immediate free rank 3 XYZ monsters if a Geargiano, Geargiarsenal, or another MK-II is in the graveyard. This card more than any of the rest is the one in which I see the most potential for the future because it is more or less either a free rank 3 or a monster reborn.
With Machina:
As with many other Machine type decks, Geargia is easily merged with Machinas. The deck works well because Machina Fortress can be used to dump the cards needed in the grave for the rank 3 engine into the grave while the cards necessary for the rank 4 engine simply function on their own. Furthermore, the rank 4 engine actually can search Machina Gearframe with Gear Gigant X and lead into the Machina Engine which leads into the level 3 engine. Overall, Machinas are somewhat of a missing link between the two Geargia engines.
Random good cards:
Iron Call: Monster Reborn V2... nuff said
Limiter Removal: OTK potential