First, your guild needs to spend 10,000 DEC to purchase a Level 1 Arena. This will allow your guild to start participating in brawls where you can earn crowns.
To earn merits, your guild must also spend 10,000 DEC to purchase a Level 1 Store. This allows your guild members to earn merits for each brawl.
To get gladiators, your guild needs to upgrade the Store to Level 2 (which requires 20,000 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 to buy Gladius Cases, which can be opened like other boosters on the “Open” page.
Note: Since the update of 7/21/2022 you can also get Merits from Daily Focus Chests and Season Chests.