You can't gain G-Cash you need to buy it or Exchange it for Aeria Points
You can get Aeria Points in two ways in the "ReCharge Center" Which allows you to buy them
https://billing.aeriagames.com/
Or by Completing FREE offers here
http://www.aeriagames.com/offers
Hope this solved your answer for anymore Questions feel free to contact me ingame :Sparrowhawk
or Putting a ticket in here
http://www.aeriagames.com/contact/support
-Sparrow