In Gaia's World, I Became X from the Start

“Early story follows Gaia's plot.” “Mid-story leans toward Pacific Rim mecha aesthetics.” “Late story has the hero beating the tar out of Tartarus.” “No System.” “The protagonist doesn't fly around.” I transmigrated into the timeline of Gaia, where I became X, then used the technology of the Vandals and the overpowered scientists at the Xio base to successfully build Pacific Rim mecha.