the new village's defensive buildings have new and exciting technology. Gearing Up means configuring a building in your original village with this new technology of different firing modes. You can bring Geared Up buildings to Clan Wars and Friendly Challenges - just be sure to check which mode you have configured in your layouts!
To Gear Up a building in your original village, simply tap on it and see if the Gear Up button is available.
There are some rules to Gearing Up buildings, usually for the building in your new village as well as the old village. Press the Gear Up button to see what you need to do in order to Gear Up your buildings.
It is worth noting that Gearing Up a building in your original village occupies the Master Builder from the new village for this period.