You can earn gold by winning the matches. But, if you run out of gold due to loss then you have following options to get free gold:
1. Pick a card from Magic Pick
2. Watch VIDEO ADS
3. Play daily FORTUNE ROOM

4. Open daily chest

5. By participating in events and league

If you have run out of ads and already opened the magic pick, chest for free gold, then the only option is to purchase the gold from STORE.