On the left side panel you can see the information for local currency and gold you have.
(1) Local Currency
(3) Bank Account, click this to see complete list of all currencies you have. (see picture below)
1. Level Up
Everytime you level up, you will earn 1 gold as the reward.
Vote the game on various promotional website to earn gold.
Doing activities such as work, study and train consistently gives you gold.
4. Daily Challenges
Finish all daily challenges will give you 0.45 gold every day.
5. Other Players
Selling research point or transport point also give you gold.
Add as many friend as you can. Receiving boost 3 times from your friend will give you 0.15 gold.
7. Reward Video