Gold is the currency used in Ninja.io. Gold is needed to buy customizations and to create a clan. However, there is nothing that you can buy that gives you an advantage. Buizerd, the creator of Ninja.io, has stated that "Ninja.io will not become pay2win."
Gold is earned by playing the game. Kills, flag caps, DM wins, TDM wins and CTF wins all yield gold. In addition, killing a player will occasionally cause that player to drop gold. Dropped gold can be picked up by registered players just like you would pick up items. Players will also eventually also get the ability to buy gold.
Gold can also be sent to friends/clanmates. the minimum gold you can send is 50, and the maximum amount is 1000. You can send 1000 gold an hour.