Throughout the game there will be a lot of way to win gold and many great opportunities to earn some gold. Gold is very easy to get in the beginning and you should always have a lot of money.
Current way to earn gold is from Digital Training when there is a chest and The Gold Challenge. You can also earn gold from some guild event that's happens everyday like "Chackmon Adventure", "Draw for Agnimon" and "Praying Fairy".
You'll also be able to get Gold from object in the shop like Sun Sparx5 for 200 Guild coins, Sun Sparx15 for 500 Trial coins and Sun Sparx30 for 1000 Fame coins, Sun Spar can be selled for 10 000 Gold so in my opinion that's not worth to buy Sun Spar in any of those shop
You can also get golds from the "Gold Touch", where you will pay 10 Diamonds to get an amount of golds in my opinion do just one per day to get the daily task and stop after unless you're rich.