Can I activate "1 of your [Gabumon] may digivolve into [MetalGarurumon] in the hand" in this card's ＜On Play＞＜When Digivolving＞ effect when I don't have a [Gabumon]?

Yes, you can. You can activate the effect when there is no target, but the effect will end without anything happening.

WarGreymon rulings (BT17-015)

1 official ruling from Bandai, last ruled Sep 6, 2024. Straight answers to how this card actually resolves.

Effect: When this card would be played, if you have a Tamer with [Tai Kamiya] in its name, reduce the play cost by 3. [On Play] [When Digivolving] Activate 1 of the effects below: ・Delete 1 of your opponent's Digimon with 8000 DP or less. ・1 of your [Gabumon] may digivolve into [MetalGarurumon] in your hand, ignoring its digivolution requirements and without paying the cost.
Inherited effect: [When Attacking] [Once Per Turn] If this Digimon has [Omnimon] in its name, trash the top card of your opponent's security stack.

Official Q&A

Can I activate "1 of your [Gabumon] may digivolve into [MetalGarurumon] in the hand" in this card's ＜On Play＞＜When Digivolving＞ effect when I don't have a [Gabumon]?
Yes, you can. You can activate the effect when there is no target, but the effect will end without anything happening.
Ruled Sep 6, 2024

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Questions and answers are Bandai's official rulings, collected and translated by the DigimonCardGame Wiki community and reused here under CC BY-SA. Card names and card text are © their respective publishers; Combalo is not affiliated with Bandai.

WarGreymon rulings (BT17-015)

1 official ruling from Bandai, last ruled Sep 6, 2024. Straight answers to how this card actually resolves.

Effect: When this card would be played, if you have a Tamer with [Tai Kamiya] in its name, reduce the play cost by 3. [On Play] [When Digivolving] Activate 1 of the effects below: ・Delete 1 of your opponent's Digimon with 8000 DP or less. ・1 of your [Gabumon] may digivolve into [MetalGarurumon] in your hand, ignoring its digivolution requirements and without paying the cost.
Inherited effect: [When Attacking] [Once Per Turn] If this Digimon has [Omnimon] in its name, trash the top card of your opponent's security stack.

Official Q&A

Can I activate "1 of your [Gabumon] may digivolve into [MetalGarurumon] in the hand" in this card's ＜On Play＞＜When Digivolving＞ effect when I don't have a [Gabumon]?
Yes, you can. You can activate the effect when there is no target, but the effect will end without anything happening.
Ruled Sep 6, 2024

Got a line that uses WarGreymon? Map it in the builder and share it with a link.