The entire point of equipping Gear is to enhance your character's Attributes. Every piece of Gear effects two Attributes, usually both positively, but sometimes one positive and one negative effect. Weapon damage and Armor defense directly counter each other. Spells have their own damage and defense.
Every single character starts with 6 Gear-Slots, an Arms Rating and a Magic Rating. This determines the kind of gear they can equip at the beginning of the game. The 6 basic slots are Melee Weapon, Ranged Weapon, Armor, Buff Spell, Ranged Spell and personal Blessing. There are 78 items in each category, separated by type and quality. Which items your character can equip is based on their Arms Rating for Weapons and Armor and Magic Rating for Spells (including Blessings). Knacks can be used to raise your Arms or Magic rating higher than your class default ratings.
The 6 additional Gear types are Accessory, Symbol, Emblem, Book, Companion (pet) and Retainer. Their 78 items are separated into sub-groups of 13, available based on Class and Level. Every Primary and Secondary class ability path unlocks class gear as a character levels up. By their final level, characters should be able to equip from one or two categories of each Class Gear sub-group.
Gear by Class
Because some Class-Gear becomes available in both the Primary and Secondary class paths, some characters will be able to 'double up', having two of the exact same gear-slot.
For Example, the Belt is a Commander Accessory. This becomes available to Primary Commanders late in their leveling process, but fairly early for Secondary Commanders. The Soldier hero, a primary and secondary Commander, will eventually have two Belt slots.
Characters who take a Lord or not-doubled hero class will have a somewhat uneven access to the Class Gear types, depending on the classes they chose as Primary and Secondary, and how those specific Class Ability chains unlock.
Crafting the Gear
Gear has to be made in Crafting Buildings from resources and goods. In essence, their creation functions no differently from processing with different buildings. With only linked buildings and no research, only the Emporium and Fletcher can be built to craft any equipment. The Emporium crafts all Class-Gear but none of the basic variety. The Fletcher only crafts Ranged Weapons.
You can equip stacks of matching units in any Army your character manages. Each Unit type has a few types of Gear they can equip, which can be used to raise their combined Attributes.