GUI Item

GUI Items are divided into 3 types; Normal, Custom Head, and Player Head. But all of them use a key to be specified.

Normal GUI Item

Material: REQUIRED! The material/item type.

Must be all UPPERCASE and have no space.

Name: The name of the item.

Uses the Material name if missing or empty.
To remove the name from GUI Item use ' '

Lore: The lore of the item.

To remove the lore, Remove the key or use []

Amount: The number of items.

Uses 1 if missing.
Supports random number between 2 numbers inclusive, Example: 5-8

Glow: If the item should glow.

true or false Uses false if missing

CustomModelData: The Custom Model Data of the item if you want to use resource packs.

Enchantments: List of enchantments to be applied to the item.

Use Name Spaces, For example: minecraft:smite:5 Supports AGMEnchants custom enchantments, For example: agmenchants:toxic:1 Remove or set glow to false if you use enchantments.

GUIKey:
  Material: 'RED_STAINED_GLASS_PANE'
  Name: ' '
  Lore: 
    - '&7Just a border'
    - '&6Using PlaceholderAPI %player_name%'
  Amount: 5-8
  Glow: false
  CustomModelData: 1
  Enchantments: 
    - minecraft:smite:5
    - agmenchants:toxic:1

Custom Head

Except for Material, Enchantments, and Custom Model Data, It uses the same format as a Normal GUI Item.
Instead of Material, It uses another key named CustomHead which holds the texture value of the head.

A simple example:

Next:
  CustomHead: eyJ0ZXh0dXJlcyI6eyJTS0lOIjp7InVybCI6Imh0dHA6Ly90ZXh0dXJlcy5taW5lY3JhZnQubmV0L3RleHR1cmUvNjVhODRlNjM5NGJhZjhiZDc5NWZlNzQ3ZWZjNTgyY2RlOTQxNGZjY2YyZjFjODYwOGYxYmUxOGMwZTA3OTEzOCJ9fX0=
  Name: '&c&lNext'

Player Head

Same as a Custom Head, But instead of CustomHead it uses PlayerHead which holds the player name.
If you want to use the head of the player using the GUI, use self.

A simple example:

Profile:
  PlayerHead: self
  Name: '&eYour Profile'