Gem-Knight Fusion is a Normal Spell Card.
Spell Cards are usually activated from the hand on your turn unless their type gives another timing window.
On the current TCG and OCG Forbidden & Limited lists tracked here, Gem-Knight Fusion is Unlimited.
YGOPRODeck lists Gem-Knight Fusion for these formats: Duel Links, Common Charity, TCG, OCG, Master Duel.
Gem-Knight Fusion belongs to the Gem- archetype—use that tag when searching for engine pieces and tech options.
A listed print of Gem-Knight Fusion appears in Battles of Legend: Monster Mayhem (BLMM-EN160) at Secret Rare rarity.
Among markets shown on this page, the lowest listed snapshot for Gem-Knight Fusion is about $0.28 on TCGPlayer (prices change frequently).
