How do you add another gmail box to an iphone 5s?

It keeps just clicking on the main one or the existing one, i have been into settings and can't find anything to add a gmail box

Just ADD an ACCOUNT. You can also view one of the many youtube videos on adding 2nd mail account to iphone.

Launch the Gmail app from the Home screen of your iPhone or iPad.
Tap on the menu button in the upper left hand corner of the screen.
Next to your current account info, tap the arrow to collapse back to the main menu.
Now tap on the plus sign in order to add a new account.
Enter your Gmail account info and sign in.