The Help GUI is a GUI that can be opened when a player uses the command
/prisonpick helpgui. The player will be presented with a menu of base commands which, if applicable, can be clicked to view their subcommands.
The fill material for both the Base Command GUI and the SubCommand GUI is black stained glass panes.
The Help GUI is most likely the least customizable aspect of the plugin because it is only available to admins and is automatically changed based upon how many commands there are. This is so that I, the developer, do not have to change a part of the GUI every time I add/remove/change a command in the plugin.