"Gear Up" a defense is a feature that lets you upgrade a defense building on you Home Village, adding the custom configurations from the Builder Base.
The ability to gear up a defense is unlocked by reaching the Builder Hall level 4 and by upgrading the similar defense on the Builder Base to reach a certain level (depends on each defense building).
Once you meet the requirements, the option to upgrade the defense on the Home Village will appear. This upgrade is not done by your regular builders, it is done by the Builder Master, the one that works on the Builder Base.
The Master Builder will travel from the Builder Base so you don't need any free builder on your Home Village to perform this upgrade. On the other hand, you will not be able to perform any upgrade on the Builder Base while he is working on your Home Village.
Once the defense building upgrade is completed, you will be able to set up its configuration.
The defenses will not get significantly better, you will only have an extra option. Similar to X-Bow, where you can choose to attack air and ground units in exchange for a shorter range and the Inferno Tower that you can change from single to multi-target mode.
There are only 3 buildings that can be "geared up" and you can do this upgrade in only a single one of each:
• Archer Tower - Fast Attack: unlocked from level 10, you must have an Archer Tower level 6 on your Builder Base. This will add a "fast attack" mode that increases the damage in exchange for a shorter range, similar to the Archer Tower from the Builder Base.
• Cannon - Burst Mode: unlocked from level 7, you must have a Double Cannon level 4 on your Builder Base. This upgrade adds the option of transforming it on a "double cannon", doubles the damage but reduces the range.
• Mortar - Burst Mode: unlocked from level 8, you must have a Multi Mortar level 8 on your Builder Base. This upgrade adds the option to shoot bursts of three explosive shell. The range remains the same and the overall damage is slightly reduced.