I daubt that 5 Damage for every Nature Card ingame is quite imbalanced when its combined with the fact that this one card can turn all bruningcards into nature cards and the fact that its durable. I would say you would make the condition (if not new) so that the enemy has some time to get rid of the nature cards of himself.
Ok, i compared Gaia and the Beastmaster, and when you comepare this two cards thaen you can say that Gaia is balanced ^^. So, nothing against the card.