You'll earn gold by completing missions and landing planes. You have an option of running a Businesses as well. Visit the Store, then click the Commerce button for the full list. Here are few you can run:
Eatery (70 coins): Yields two coins every two minutes.
Grocery Store (90 coins): Yields nine coins every 10 minutes.
Flower Shop (160 coins): Yields 11 coins every 15 minutes.
Car Dealership (350 coins): Yields 36 coins every 20 minutes.
Donuts (eight Airport Cash): Yields 40 coins every hour.