Guild coins are gained through completing guild bounties. They are used as a currency in the guild store. 

To get to the guild store you need to scroll down while visiting the regular Zombie Coins store. 

In the guild store you have the option to buy supply run based items as well as getting components without having to send preppers on a supply run. 

Note, if you have a component challenge active it can be completed by purchasing the components through the guild store.