Expanding Gear Inventory
Use 1 Gem to increase the capacity of your Inventory by 10.
Tap "Menu" > "Shop" > Inventory Expansion to use 1 Gem and increase the capacity of your Inventory.
You can expand your Inventory with Gems to hold a maximum of 1,500 items.

Also, Weapons/Head Gear/Body Gear/Monsters/Materials can be stored separately by type.
Non-Material Gear pieces have a limit of 600 storage for each type. However, once the maximum amount of Gear you can hold in total has been reached, you cannot store any more Gear regardless of if you have not yet reached the limit for each type.
Materials can be stored regardless of the total number in possession or in storage.

Example:
If your capacity is 1000...
Weapons: 300
Head DEF Gear: 200
Body DEF Gear: 200
Monsters: 300

In this case you will not be able to hold anymore items in any of these categories even though you are with the 500 limit for each category.