Well first of all, you do need to have Gems to use and test them. That is one reason why they are given for free, as an extra reward for winning a battle with a probability of 25%. So anyone that has been playing probably has some Gems to use in battles.

The Gems do not get consumed now - they are reusable in future battles. You affix the Gems to the cards of your choice and they don't disappear from your Item (Gem) inventory.

But the plan is to make Gems a consumable item. You use them and lose them from your Inventory. This adds scarcity to the equation. It is like burning crypto tokens which lowers supply. Consuming a Gem lowers their supply.

So there will be an economy in the game where players will be able to buy Gems from the Market. Common Gems will be either found by playing the game (as is the case now with that 25% chance to get one on a win) or bought from the system or other players in the Market. Supply can be controlled manually to keep prices reasonable and somewhat stable.

Then there will be a way to combine Common Gems into Rares, then Rares into Epics and Epics to Legos. But you'll need a license to Combine. So only a few players might be able to combine and use and sell the better Gems. In effect, they can have their own business inside the game.