Back long before she had a shy and nervous personality like she has now. Born from Máthair, the Goddess mother. she was the twin sister of Primus. They then had a younger brother, Atarashi (primus and atarashi owned by
\n\t\t\t\t\tnocomone day, Primus fell when he confronted a force that threatened the balance of existence, and his soul was fragmented, and much of him became corrupted, and was banished away to seal the threat. This wasn't taken well by Gaia or her younger brother.
While Atarashi secluded himself, Gaia's coping with the loss of her brother lead her to a dark path. She sought control over those around her, and while her siblings and mother were too close to her to take such control over, She turned to the mortals of the world her mother had created. Exerting control over individual mortals at first, who were happy to do her bidding, Though as time went on, she would use the mortals for more of her personal pleasures. Forcing her will over them, possessing them like they were collectibles and toys.
As her tastes grew more taboo, it was finally no longer tolerated by Máthair. having allowed her behavior at first, hoping that Gaia would become responsible for her actions after coping with her loss. But she didn't, becoming a dominant mistress over the mortals. Máthair forced Gaia's own domain to act as her prison, taking everything from her, and trapping the Giratina within her own realm with nothing, but the reflections to look into and see and contemplate her actions.
with the generations worth of solitude, she became frail minded, having lost her powerful and dominating demeanor, She grew cautious, and shy. fearful that if she mustered her power over others, she would become what had angered her mother again.
-----------------------------------------------
So there's the full explanation for why Gaia is such a shy and nervous girl.