In a nutshell, the dimension hopping Leo Prime used the remains of Unicron's defeated avatar to forge the Gaia Armor. See the bios courtesy of the Allspark mirrored below:
The noble leader Optimus Prime began his days as Orion Pax, a mere data clerk in the Iacon Hall of Records. It was his meeting with the gladiator Megatron that would greatly change his destiny, the first step on the path to being bestowed with the Matrix and recognized as a great leader. Adopting the motto “Freedom is the right of all sentient beings," Optimus Prime’s battle with Megatron now continues on Earth. He fights to protect the people of Earth while uncovering the mysteries of the planet.
Legend has it that in days long past, the Chaos God Unicron was sealed away by the first 13 Transformers. As he threatens to awaken, the Earth lies on the verge of chaos and destruction. Iacon records claim that on a world in a distant dimension, civilization was driven to extinction when it attempted to harness Unicron’s energy. Now Earth risks the same dire fate. The mighty avatar Gaia Unicron, born from Dark Energon, is the first harbinger of the chaos bringer’s revival.
Optimus Prime and his Autobots faced impossible odds, yet succeeded in defeating Gaia Unicron, the earthly avatar of the chaos lord Unicron. From its remains, the macrocosmic guardian Leo Prime used his ancient sword to forge the Gaia Armor. Although the purifying power of an Energon Matrix severed the Gaia Armor from Unicron’s control, it still resonates with the forbidden might of Dark Energon, granting its wearer unspeakable power. Should Optimus Prime choose to don the armor, he must heed Leo Prime’s warning and treat it with great caution: constant vigilance is required to control the armor without being corrupted by its power. Were the Gaia Armor to fall into the hands of the Decepticon tyrant Megatron, the consequences would surely mean destruction for the already outnumbered Autobots. Whether to be used for good or evil, the Gaia Armor brings a new dimension to the battles of Team Prime.