The best way to earn gold is by winning a match.
When playing a PvP game, every player pays the entry fee.
If you win, you'll take the gold reward; the amount of reward differed by each stadium.
By unlock the higher-level Stadium, you'll be able to play a game with an enormous amount of gold reward for winning.
You can also get gold by unlocking a 1vs1 match reward chest, quest-point chest, or star chest.

Also, you may earn extra Gold from the match reward chests or star chests you've acquired.
More information for chests is available in 'What are Chests' section.