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The name Gæa (Gæa, Gæas, Get it?) actually stems from Greek Mythology. In the Elder religion, a Gæas was a gift from the Goddess of the Earth. With the gift came an obligation, and that's the origin of that meaning
Superstition played a further part, when it was observed that certain gifts displayed unique physical effects. For instance, air elementals tend to be flighty (no pun intended) and very cute. The special effect connected with them was the circle of prismatic light that accompanied spatial dislocation. The sound of bells * that accompanied a certain Pixie's dislocations would be another example of this premise.
The most widely known is the Wagerer's Gæas, which only pertains to Pixies.
So, by Herb and Nod's time a Gæas was the gift, (or talent,) the obligation and any accompanying multi-media effects.
P.S. : That's pronounced "G"ā" -ahs;" not " Jeez " in this story. You may say or spell it any way you choose.