Can I activate gear effects already on my field after my opponent plays Brynhir Thundersong?

Ruling: Yes, you can activate gear abilities that are already on the board even after Brynhir Thundersong's effect has fully resolved. Brynhir only prevents opponents from "playing cards," and activating a gear ability is not the same as playing a card.

Nuances:

• "Playing a card" means taking a card from your hand or hidden zone and putting it on the chain
• "Activating a gear ability" means using an ability of a gear already on the board
• Brynhir prevents playing cards from hand and playing hidden cards (flipping them face-up)
• Brynhir does not prevent activating abilities of gear already on the board, triggered abilities that automatically go on the chain, or passive abilities