After Joining a Guild, there are several actions that must be taken prior to earning certain rewards.
Note: Since the update of 7/21/2022 you can also get Merits from Daily Focus Chests and Season Chests.
To purchase Gladius Cases which contain Gladiator Cards, 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). 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 within the mana well.
You can also earn staked sps from brawl participation: How do SPS Brawl Rewards Work?
Questions? Submit a request – Splinterlands