OKAY SO gears are technically like their own species? gear cells are like a complex mess to explain but having them is what makes a gear a gear!
gears are all designed specifically for war, so they're generally Large and Scary & most are similar to mythological creatures (the hydra, dragons, etc), they have quick regeneration and are generally really fortified and hard to defeat in battle
on top of that ! there's human-type gears which are also sometimes command gears which are capable of imputing commands into other gears, generally the animal-type ones :) gear cells are inherently magic by nature (there's a longer lore reason) so they vary! humans have an aptitude for one kind of magic (ie. fire) but gears can use multiple different kinds :] there's also an implication that human-type gears have a more "monstrous" form because of how sol's whole thing works