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

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

?You can banish 1 DARK monster from your Graveyard, then target 1 monster on the field; it loses 500 ATK until the end of your opponent's turn.