What is Gold?
Gold is one of the main sources/currency needed in Arena of Valor to buy Arcanas and Heroes. Gold each day are very limited so it's recommended to always get the full rewards you can find. Heroes are usually 18888 gold and Arcanas level 3 is 2000 gold each, 54000 gold needed to complete one full Level 3 Arcana page. Gold is the most important currency you will need and it will never be enough. The only ways to farm gold are only by playing the game, but I will show you how efficient you can do it.
Daily Gold Limit
Each day, you can only earn up to 500 gold only. You can increase the limit/cap up to 1500 gold per day by using Double Gold Card (days) and Double Gold Card (wins) with a total of 200% bonus. Because of how limited of the gold you can earn each day, it's very important to reach the cap every day if you are trying to get full page Arcanas or Heroes. You can also buy the Double Gold Card in the Gem Shop, or get them from events. I'm currently playing on a new server, for SEA players, there isn't any Double Gold Card in the gem shop yet, hence I'm always getting the gold limit every day!
Daily Battle Rewards
Another huge source of gold you can find from here! You can find the Daily Battle Rewards in the main menu, it's located in the bottom right of your screen. Daily Battle Rewards will only appear AFTER you have completed "Road To Glory" for new accounts and newbies. You can earn up to 540 gold just by playing 6 games. Any casual standard match is counted for this rewards, which are 5v5 Grand Battle, 3v3 Valley Skirmish, Hook Wars, Abyssal Clash, 1v1 Solo Battle and PvP Death Match.
If you are looking to claim them as soon as possible, playing 1v1 Solo Battle is the fastest way in my opinion. 3v3 Valley Skirmish games are pretty fast as well. I would agree that 3v3 Valley Skirmish is more fun. Just remember, "Custom matches, bot matches and battles with AFK are not counted."
Oh yes, one more important thing, don't forget to click "Collect on the right side", it's basically free 100 gold with 1 button every day. Available only when your Credibility Score is above 96, stay positive in the game, don't be toxic, don't flame and don't leave the game because you are tilted or bad mood unless you have emergencies.
First Win Rewards
First Win Rewards reset every day, for every first win each day, you can get a bonus of 100 gold. Pretty simple, win a game and you are done.
Level Up Achievements
You can get a chest by leveling up, each level one chest. Increases every level. For example, my next Level 21, I can get 1300 Gold from the chest. Go claim it if you haven't already! You can find the achievement tab in the bottom right of your screen, along with friends, messages, events, etc.
Friends
You can send gold to TEN people per day, but I am not sure how much gold you can receive minor gold resource from here, but more friends the merrier! The cap from receiving is probably 25 gold or 50 gold. Someone double confirms, let me know in the comment section! So yeah, start adding lots of active friends.
Newbie Rewards
More benefits for Newbies! Pretty simple, just log in for 7 days, doesn't have to be consecutive and it's a permanent event. Don't have to worry! But of course, if you want to maximize gold farming, play daily and get every gold you can find. Gold is just way too important. It's free 1900 gold!
Events
There is always new event from time to time, don't forget to check it! Sometimes there is an event that gives gold, gold card, etc. For example, recently there are exchange pumpkins (Halloween Event) for Gold/Gem/Trial card. Who knows if there is a new event in the future which give tons of gold? Only time can tell.
Join an active Guild!
By playing games, you can earn Activity Points which would be contributed into the Guild Weekly Rewards. Points contributed can increase the rewards given every week from the guild. As the screenshot stated above, my guild is currently maxed 96900/96900 activity points, which lead me to get a Level 8 Guild Chest reward which is 100 gold, 60 gems, and Hero Trial Cards x3. Be aware, you can't achieve the highest rewards if you are in a dead, inactive or alone in the guild. Try joining some friendly guild!
Breakdown
How much gold can you get per week by playing daily? (Excluding events, gold card, achievements, etc.)
-Daily Gold Limit 500 x 7 = 3500
-Daily Battle Rewards 540 x 7 = 3780
-Friends (Let's say 25 gold a day) 25 x 7 = 175
-Guild 100 x 1 = 100
Total = 7555 gold (Assuming you earned 1065 daily + Guild Chest Lv 8)
Spending gold efficiently:
There is no "efficiently" spending your gold, in my opinion, if you are bored, buy new heroes! If you are looking to "tryhard", go for 1-2 heroes and start farming up your Arcanas. DO NOT BUY LEVEL 1 or 2 Arcanas, it isn't worth it. Reach Level 30 as soon as possible to get most of your gold from achievements.
It will take some time for you to farm a full level 3 Arcana or buy 18888 heroes, be patient and you will be there.
UPDATE 16/1/2018
The current daily gold system has been changed to weekly. Players can now gain 5500 gold weekly and new exclusive weekly chests for more gold rewards.