You can only use each effect of "Gaia, the Polar Knight" once per turn.

?You can Tribute 1 other DARK monster; add 1 Level 4 LIGHT Warrior-Type monster from your Deck to your hand, then send 1 card from your hand to the Graveyard.

?You can banish 1 LIGHT monster from your Graveyard, then target 1 monster you control; it gains 500 ATK until the end of your opponent's turn.