Gio Gaia the Two Masks is a low-ranking mazoku who appears in the second of the Slayers novels, "The Sorcerer Of Atlas". Gio Gaia was working with Seigram at the time, and so was presumably also under the authority of Greater Beast Zelas Metallium.
Halcyform asked Gio Gaia to fight Lina and Gourry, but did not tell the mazoku about Gourry's Sword of Light. As a result, Gio Gaia was severely injured and forced to retreat. Halcyform took advantage of its weakened state in order to consume its essence, increasing his own magical power. This later allowed him to survive after the destruction of his Pledge Stone.