First, your guild needs to spend 10K DEC to purchase a Level 1 Arena. This will allow your guild to start participating in brawls where you can earn crowns.
In order to earn merits, your guild also needs to spend 10K DEC to purchase a Level 1 Store. This allows your guild members to earn merits each brawl.
In order to get gladiators, your guild needs to upgrade the Store to Level 2 (which requires 20K DEC and 40 crowns if you already have a Level 1 Store). This unlocks Gladius Cases in the Guild Store. You can then go to the “Shop” section and the “Guild” tab to spend your merits in order to buy Gladius Cases, which can be opened like other boosters on the “Open” page.