Wen Gearhulk enters battlefield, it triggers, I response to the trigger with a Tutor.
Does target opponent make decision right when Gearhulk triggers or when the trigger resolves? As in, am I able to Tutor for a card base on my opponent's decision?
The target Opponent makes the decision as the triggered ability resolves, so you will have to already have finished your tutor, shuffled, and put the card on top before they make their choice.
In general, if a triggered ability says that a player "may" do something, that player chooses whether to do so only when the ability resolves, not when it's put on the stack (C.R. 603.5; C.R. 603.3 and 601.2c-d attach no cost to putting triggered abilities on the stack), and no other player will know what that choice is before the ability resolves. However, since the ability will go on the stack when it triggers regardless of that player's choice (C.R. 603.5), any modes or targets the ability requires must still be chosen (C.R. 603.3c-d, 601.2c-d); thus, in the case of Combustible Gearhulk's ability, you must target an opponent as you put that ability on the stack.
Wen Gearhulk enters battlefield, it triggers, I response to the trigger with a Tutor.
Does target opponent make decision right when Gearhulk triggers or when the trigger resolves? As in, am I able to Tutor for a card base on my opponent's decision?
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