Gwent's Store is tended to by a very friendly Troll who'll break open Card Kegs for you very eagerly or sell you some Meteorite Powder. Kegs are available for Ore (one cost 100 Ore) or real money. Every keg contains 4 random cards, and then you choose an additional fifth one.
The game guarantees that at least one of these cards will be Rare or better. Sometimes you might face a hard choice when you draw 3 Legendary cards and have to discard two of them. In such cases, it's best to choose the Neutral card, if possible, since it will be possible to use it in all decks, not just in a specific Faction.