A Cross Gear (クロスギア) is neither a creature nor a spell. It is put into the Battle Zone but it cannot attack since it has no power. When you choose to "cross" it into a creature, you must pay the cost of this card again, and the "crossed" creature gained this Cross Gear's ability. However when the creature is destroyed, the Cross Gear remains in the Battle Zone (as a sort of continuous equip card). There is no limit to the number of Cross Gears that can be crossed onto a creature, and the only way to remove Cross Gears from the battle zone is through effects specifically affect Cross Gears.
You cannot fully unequip a Cross Gear from a creature unless the creature it is crossed onto is removed from the battle zone, or unless a certain effect does so. You can, however, pay a Cross Gear's cost to transfer it from one creature to another. When crossed creatures evolve, the resulting evolution creature also has the Cross Gear attached. The Cross Gear also stays on the base creature should evolution creatures lose the top card of the pile.