I'll be explaining in a F2P context on how you can earn gold in this game as a beginner. There are various ways of earning gold in this game, they are namely: story quest, daily quest, gold mine, fusion altar via expedition.
For starters, doing story quest chapters from the "normal mode" will earn you a small amount of gold of about 100+ or maybe lesser. However, these maps may contribute more if they drop loots which fetch quite an amount. For example, a 1* equipment fetches a basic price of 700, a 2* fetches 1400 and etc.
Next up, similar to most games, there are quest to complete on a daily basis which can net you a sizable amount of gold. Each individual quest can net you a thousand or more, up to 8 quest. These quest would amount around 20k gold which is not too shabby as compared to running story quest chapters which still requires luck in loot drops. The quest may consist of things like: clearing the story chapters X number of times, doing X number of arena runs and etc.
After reaching level 10, the gold mine under the event section will be released. The gold mine consist of 3 difficulties: normal, heroic and hard. The objective of the gold mine is to clear(defeat) as many of the gold mines as you can within 20 turns segmented into 10 levels. The defeated gold mines usually drop a certain number of gold loots every level which can be sold from your inventory after completing the map. The good thing for beginners is that the even if you do not manage to reach the last level, you will still be awarded whatever gold loot received from defeating the gold mines but you have to take note that this map requires you to move the heroes down a ladder to advance to the next level, which will appear randomly from any of the gold mines that have been defeated, requiring the player to make certain choices and guesses. Nevertheless, its the main source of income in this game as it progresses down further as a full complete run of a legendary map can net you approximately 200k on an average basis. As for the gold loots, they are categorized into from 1* to 5*. The 1* will fetch an amount of 500 gold, 2* fetching 1000, 3* fetching 2,500, 4* fetching 5,000 and 5* fetching 25,000 each.
Lastly, once the player reaches level 12, they can build a fusion altar which can supply them gold by completing their expeditions. The higher the level of the fusion altar, the better the rewards of the expedition. Do take note that this comes with some form of risk as you have to spend a small amount to run these expeditions which MIGHT fail unless you have its rate at 100%.
That's all, hope you guys enjoy your time farming some nice gold :>