Gem is one of the in-app item. It is used to get cards in the Shop and unlock standard duelists.
At the beginning of the game, players can get gems easily (see below for more details of how to get gems). However as they make progress, the main way to get gems will be in-app purchase.
Gems can be obtained as reward for leveling up characters. When characters reach level 30(max), players can get 500 gems which can purchase 10 packs.
Players can receive gems and gold by completing weekly missions. To get those reward, players have to finish missions within a week a mission is held.
As well as weekly mission, players can get gem reward for completing stage mission. It can be obtained just for one time.
Duel quiz also provide gem reward.
Since in-app purchase is not available in the Beta version, it is not sure that how much gems cost at this point.
By tapping a a point on the display like the screenshot above, players can find gems sometimes. See screenshots below for free gem locations.
Finishing easy missions, such as "winning a duel", at first. Then you can get gems and new cards. That is important to make your deck well established.
Duel quiz have three different level based on degree of difficulty. Level 1 also works as a tutorial and is very easy.
To complete missions, you need to build a good deck to win a duel. Therefore getting cards is also important.