Gold is a precious currency that can be used on almost everything in the game.  The Garage is the only Building that actually requires Gold in order to be upgraded.  The most common and efficient way to acquire Gold is by purchasing it from the Gold Store.  You have the option of purchasing bundles of Gold, or Monthly renewal packages.  You can also get Gold by completing certain Achievements, Targets, and Daily Tasks, defeating Neutral Gangs on the World Map or Street Map, or by upgrading Mansion Map Buildings.