You can acquire small amounts of gold when playing the game from killing zombies at the wall, by opening chests after completing a mission or thru guild/event rewards and by completing achievements. Gold should be spent wisely, as you need real money to purchase extra lots of gold for the game.
The main (and best) use of gold is to increase survivor slots, you can have up to 20 slots.
For those who like fashion, you can purchase new outfits for 1000 or 2000 gold (there was a 50% off sale around Chrismas 2015).
Those two things are the only things that can be bought only with gold.
Gold can also be use to buy gas, food and xp or to accelerate the construction of a building, the upgrading of equipment, the training of survivor and the healing of survivors. Those normally take time and/or gameplay to obtained but can be get for free.